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  <header>
    <copyright>
      <year>1996</year><year>2018</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at

          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.

    </legalnotice>

    <title>kernel</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
  </header>
  <app>kernel</app>
  <appsummary>The Kernel application.</appsummary>
  <description>
    <p>The Kernel application has all the code necessary to run
      the Erlang runtime system: file servers, code servers,
      and so on.</p>
    <p>The Kernel application is the first application started. It is
      mandatory in the sense that the minimal system based on
      Erlang/OTP consists of Kernel and STDLIB. Kernel
      contains the following functional areas:</p>
    <list type="bulleted">
      <item>Start, stop, supervision, configuration, and distribution of applications</item>
      <item>Code loading</item>
      <item>Logging</item>
      <item>Global name service</item>
      <item>Supervision of Erlang/OTP</item>
      <item>Communication with sockets</item>
      <item>Operating system interface</item>
    </list>
  </description>

  <section>
    <title>Logger Handlers</title>
    <p>Two standard logger handlers are defined in
      the Kernel application. These are described in the
      <seealso marker="logger_chapter">Kernel User's Guide</seealso>,
      and in the <seealso marker="logger_std_h"><c>logger_std_h(3)</c></seealso>
      and <seealso marker="logger_disk_log_h"><c>logger_disk_log_h(3)</c>
      </seealso> manual pages.</p>
  </section>

  <section>
      <marker id="erl_signal_server"/>
      <title>OS Signal Event Handler</title>
      <p>Asynchronous OS signals may be subscribed to via the Kernel applications event manager
          (see <seealso marker="doc/design_principles:des_princ">OTP Design Principles</seealso> and
          <seealso marker="stdlib:gen_event"><c>gen_event(3)</c></seealso>) registered as <c>erl_signal_server</c>.
          A default signal handler is installed which handles the following signals:</p>
          <taglist>
              <tag><c>sigusr1</c></tag>
              <item><p>The default handler will halt Erlang and produce a crashdump
                      with slogan "Received SIGUSR1".
                      This is equivalent to calling <c>erlang:halt("Received SIGUSR1")</c>.
              </p></item>

              <tag><c>sigquit</c></tag>
              <item><p>The default handler will halt Erlang immediately.
                      This is equivalent to calling <c>erlang:halt()</c>.
              </p></item>

              <tag><c>sigterm</c></tag>
              <item><p>The default handler will terminate Erlang normally.
                      This is equivalent to calling <c>init:stop()</c>.
              </p></item>
          </taglist>

          <section>
              <title>Events</title>
              <p>Any event handler added to <c>erl_signal_server</c> must handle the following events.</p>
              <taglist>
                  <tag><c>sighup</c></tag>
                  <item><p>Hangup detected on controlling terminal or death of controlling process</p></item>
                  <tag><c>sigquit</c></tag>
                  <item><p>Quit from keyboard</p></item>
                  <tag><c>sigabrt</c></tag>
                  <item><p>Abort signal from abort</p></item>
                  <tag><c>sigalrm</c></tag>
                  <item><p>Timer signal from alarm</p></item>
                  <tag><c>sigterm</c></tag>
                  <item><p>Termination signal</p></item>
                  <tag><c>sigusr1</c></tag>
                  <item><p>User-defined signal 1</p></item>
                  <tag><c>sigusr2</c></tag>
                  <item><p>User-defined signal 2</p></item>
                  <tag><c>sigchld</c></tag>
                  <item><p>Child process stopped or terminated</p></item>
                  <tag><c>sigstop</c></tag>
                  <item><p>Stop process</p></item>
                  <tag><c>sigtstp</c></tag>
                  <item><p>Stop typed at terminal</p></item>
              </taglist>

              <p>Setting OS signals are described in <seealso marker="os#set_signal/2"><c>os:set_signal/2</c></seealso>.</p>
          </section>
  </section>

  <section>
    <marker id="configuration"/>
    <title>Configuration</title>
    <p>The following configuration parameters are defined for the Kernel
      application. For more information about configuration parameters,
    see file <seealso marker="app"><c>app(4)</c></seealso>.</p>
    <taglist>
      <tag><c>distributed = [Distrib]</c></tag>
      <item>
        <p>Specifies which applications that are distributed and on which
          nodes they are allowed to execute. In this parameter:</p>
        <list type="bulleted">
          <item><c>Distrib = {App,Nodes} | {App,Time,Nodes}</c></item>
          <item><c>App = atom()</c></item>
          <item><c>Time = integer()>0</c></item>
          <item><c>Nodes = [node() | {node(),...,node()}]</c></item>
        </list>
        <p>The parameter is described in
	  <seealso marker="application#load/2"><c>application:load/2</c></seealso>.</p>
      </item>
      <tag><c>dist_auto_connect = Value</c></tag>
      <item>
        <p>Specifies when nodes are automatically connected. If
          this parameter is not specified, a node is always
          automatically connected, for example, when a message is to be sent to
          that node. <c>Value</c> is one of:</p>
        <taglist>
          <tag><c>never</c></tag>
          <item><p>Connections are never automatically established, they
           must be explicitly connected. See
	   <seealso marker="net_kernel"><c>net_kernel(3)</c></seealso>.</p></item>
          <tag><c>once</c></tag>
          <item><p>Connections are established automatically, but only
           once per node. If a node goes down, it must thereafter be
           explicitly connected. See
	   <seealso marker="net_kernel"><c>net_kernel(3)</c></seealso>.</p></item>
        </taglist>
      </item>
      <tag><c>permissions = [Perm]</c></tag>
      <item>
        <p>Specifies the default permission for applications when they
          are started. In this parameter:</p>
        <list type="bulleted">
          <item><c>Perm = {ApplName,Bool}</c></item>
          <item><c>ApplName = atom()</c></item>
          <item><c>Bool = boolean()</c></item>
        </list>
        <p>Permissions are described in
	  <seealso marker="application#permit/2"><c>application:permit/2</c></seealso>.</p>
      </item>
      <tag><marker id="logger"/><c>logger = [Config]</c></tag>
      <item>
        <p>Specifies the configuration
	  for <seealso marker="logger">Logger</seealso>, except the
	  primary log level, which is specified
	  with <seealso marker="#logger_level"><c>logger_level</c></seealso>,
	  and the compatibility
	  with <seealso marker="sasl:error_logging">SASL Error
	  Logging</seealso>, which is specified
	  with <seealso marker="#logger_sasl_compatible">
	    <c>logger_sasl_compatible</c></seealso>.</p>
        <p>The <c>logger </c> parameter is described in
          section <seealso marker="logger_chapter#logger_parameter">
	    Logging</seealso> in the Kernel User's Guide.</p>
      </item>
      <tag><marker id="logger_level"/><c>logger_level = Level</c></tag>
      <item>
	<p>Specifies the primary log level for Logger. Log events with
	  the same, or a more severe level, pass through the primary
	  log level check. See
	  section <seealso marker="logger_chapter">Logging</seealso>
	  in the Kernel User's Guide for more information about Logger
	  and log levels.</p>
	<p><c>Level = emergency | alert | critical | error | warning |
	    notice | info | debug | all | none</c></p>
	<p>To change the primary log level at runtime, use
          <seealso marker="logger#set_primary_config/2">
            <c>logger:set_primary_config(level, Level)</c></seealso>.</p>
	<p>Defaults to <c>notice</c>.</p>
      </item>
      <tag><marker id="logger_sasl_compatible"/>
	<c>logger_sasl_compatible = true | false</c></tag>
      <item>
	<p>Specifies if Logger behaves backwards compatible with the
	  SASL error logging functionality from releases prior to
	  Erlang/OTP 21.0.</p>
	<p>If this parameter is set to <c>true</c>, the default Logger
	  handler does not log any progress-, crash-, or supervisor
	  reports. If the SASL application is then started, it adds a
	  Logger handler named <c>sasl</c>, which logs these events
	  according to values of the SASL configuration
	  parameter <c>sasl_error_logger</c>
	  and <c>sasl_errlog_type</c>.</p>
	<p>See section
	  <seealso marker="sasl:sasl_app#deprecated_error_logger_config">
	    Deprecated Error Logger Event Handlers and
	 Configuration</seealso> in the sasl(6) manual page for
	 information about the SASL configuration parameters.</p>
	<p>See section <seealso marker="sasl:error_logging">SASL Error
	  Logging</seealso> in the SASL User's Guide, and
	  section <seealso marker="logger_chapter#compatibility">Backwards
	  Compatibility with error_logger</seealso> in the Kernel
	  User's Guide for information about the SASL error logging
	  functionality, and how Logger can be backwards compatible
	  with this.</p>
	<p>Defaults to <c>false</c>.</p>
	<note>
	  <p>If this parameter is set to <c>true</c>,
	    <c>sasl_errlog_type</c> indicates that progress reports
	    shall be logged, and the configured primary log level
	    is <c>notice</c> or more severe, then SASL automatically
	    sets the primary log level to <c>info</c>. That is, this
	    setting can potentially overwrite the value of the Kernel
	    configuration parameter <c>logger_level</c>. This is to
	    allow progress reports, which have log level <c>info</c>,
	    to be forwarded to the handlers.</p>
	</note>
      </item>
      <tag><c>global_groups = [GroupTuple]</c></tag>
      <item>
        <marker id="global_groups"></marker>
        <p>Defines global groups, see
	  <seealso marker="global_group"><c>global_group(3)</c></seealso>.
	In this parameter:</p>
        <list type="bulleted">
          <item><p><c>GroupTuple = {GroupName, [Node]} | {GroupName, PublishType, [Node]}</c></p></item>
          <item><p><c>GroupName = atom()</c></p></item>
          <item><p><c>PublishType = normal | hidden</c></p></item>
          <item><p><c>Node = node()</c></p></item>
        </list>
      </item>
      <tag><c>inet_default_connect_options = [{Opt, Val}]</c></tag>
      <item>
        <p>Specifies default options for <c>connect</c> sockets,
          see <seealso marker="inet"><c>inet(3)</c></seealso>.</p>
      </item>
      <tag><c>inet_default_listen_options = [{Opt, Val}]</c></tag>
      <item>
        <p>Specifies default options for <c>listen</c> (and
          <c>accept</c>) sockets, see <seealso marker="inet"><c>inet(3)</c></seealso>.</p>
      </item>
      <tag><c>{inet_dist_use_interface, ip_address()}</c></tag>
      <item>
        <p>If the host of an Erlang node has many network interfaces,
          this parameter specifies which one to listen on. For the type definition
	  of <c>ip_address()</c>,
	  see <seealso marker="inet"><c>inet(3)</c></seealso>.</p>
      </item>
      <tag><c>{inet_dist_listen_min, First}</c> and <c>{inet_dist_listen_max, Last}</c></tag>
      <item>
        <p>Defines the <c>First..Last</c> port range for the listener
          socket of a distributed Erlang node.</p>
      </item>
      <tag><c>{inet_dist_listen_options, Opts}</c></tag>
      <item>
	<marker id="inet_dist_listen_options"></marker>
        <p>Defines a list of extra socket options to be used when opening the
	  listening socket for a distributed Erlang node.
	  See <seealso marker="gen_tcp#listen/2"><c>gen_tcp:listen/2</c></seealso>.</p>
      </item>
      <tag><c>{inet_dist_connect_options, Opts}</c></tag>
      <item>
	<marker id="inet_dist_connect_options"></marker>
        <p>Defines a list of extra socket options to be used when connecting to
	  other distributed Erlang nodes.
	  See <seealso marker="gen_tcp#connect/4"><c>gen_tcp:connect/4</c></seealso>.</p>
      </item>
      <tag><c>inet_parse_error_log = silent</c></tag>
      <item>
        <p>If set, no log events are issued when erroneous lines are
          found and skipped in the various Inet configuration
          files.</p>
      </item>
      <tag><c>inetrc = Filename</c></tag>
      <item>
        <p>The name (string) of an Inet user configuration file. For details,
	see section
	<seealso marker="erts:inet_cfg"><c>Inet Configuration</c></seealso>
	in the ERTS User's Guide.</p>
      </item>
      <tag><c>net_setuptime = SetupTime</c></tag>
      <item>
        <marker id="net_setuptime"></marker>
        <p><c>SetupTime</c> must be a positive integer or floating point
          number, and is interpreted as the maximum allowed time
          for each network operation during connection setup to another
          Erlang node. The maximum allowed value is <c>120</c>. If higher values
          are specified, <c>120</c> is used. Default is 7 seconds if the variable
          is not specified, or if the value is incorrect (for example, not a number).</p>
        <p>Notice that this value does not limit the total connection
          setup time, but rather each individual network operation during
          the connection setup and handshake.</p>
      </item>
      <tag><c>net_ticktime = TickTime</c></tag>
      <item>
        <marker id="net_ticktime"></marker>
        <p>Specifies the <c>net_kernel</c> tick time in seconds. This is the
	approximate time a connected node may be unresponsive until it is
	considered down and thereby disconnected.</p>
	<p>Once every <c>TickTime/4</c> seconds, each connected node is ticked
	if nothing has been sent to it during that last <c>TickTime/4</c>
	interval. A tick is a small package sent on the connection. A connected
	node is considered to be down if no ticks or payload packages have been
	received during the last four <c>TickTime/4</c> intervals. This ensures
	that nodes that are not responding, for reasons such as hardware errors,
	are considered to be down.</p>
        <p>As the availability is only checked every <c>TickTime/4</c> seconds,
	the actual time <c>T</c> a node have been unresponsive when
	detected may vary between <c>MinT</c> and <c>MaxT</c>,
	where:</p>
        <code type="none">
MinT = TickTime - TickTime / 4
MaxT = TickTime + TickTime / 4</code>
        <p><c>TickTime</c> defaults to <c>60</c> seconds. Thus,
        <c><![CDATA[45 < T < 75]]></c> seconds.</p>
        <p>Notice that <em>all</em> communicating nodes are to have the
	<em>same</em> <c>TickTime</c> value specified, as it determines both the
	frequency of outgoing ticks and the expected frequency of incominging
	ticks.</p>
        <p>Normally, a terminating node is detected immediately by the transport
	protocol (like TCP/IP).</p>
      </item>
      <tag><c>shutdown_timeout = integer() | infinity</c></tag>
      <item>
        <p>Specifies the time <c>application_controller</c> waits
          for an application to terminate during node shutdown. If the
          timer expires, <c>application_controller</c> brutally
          kills <c>application_master</c> of the hanging
          application. If this parameter is undefined, it defaults
          to <c>infinity</c>.</p>
      </item>
      <tag><c>sync_nodes_mandatory = [NodeName]</c></tag>
      <item>
        <p>Specifies which other nodes that <em>must</em> be alive
          for this node to start properly. If some node in the list
          does not start within the specified time, this node does not
          start either. If this parameter is undefined, it defaults to
          <c>[]</c>.</p>
      </item>
      <tag><c>sync_nodes_optional = [NodeName]</c></tag>
      <item>
        <p>Specifies which other nodes that <em>can</em> be alive
          for this node to start properly. If some node in this list
          does not start within the specified time, this node starts
          anyway. If this parameter is undefined, it defaults to
          the empty list.</p>
      </item>
      <tag><c>sync_nodes_timeout = integer() | infinity</c></tag>
      <item>
        <p>Specifies the time (in milliseconds) that this node
          waits for the mandatory and optional nodes to start. If
          this parameter is undefined, no node synchronization is
          performed. This option ensures that <c>global</c> is
          synchronized.</p>
      </item>
      <tag><c>start_dist_ac = true | false</c></tag>
      <item>
        <p>Starts the <c>dist_ac</c> server if the parameter is
          <c>true</c>. This parameter is to be set to <c>true</c> for
          systems using distributed applications.</p>
        <p>Defaults to <c>false</c>.  If this parameter is
          undefined, the server is started if parameter
          <c>distributed</c> is set.</p>
      </item>
      <tag><c>start_boot_server = true | false</c></tag>
      <item>
        <p>Starts the <c>boot_server</c> if the parameter is <c>true</c>
          (see <seealso marker="erl_boot_server"><c>erl_boot_server(3)</c></seealso>).
	  This parameter is to be set to <c>true</c> in an embedded system
	  using this service.</p>
        <p>Defaults to <c>false</c>.</p>
      </item>
      <tag><c>boot_server_slaves = [SlaveIP]</c></tag>
      <item>
        <p>If configuration parameter <c>start_boot_server</c> is
          <c>true</c>, this parameter can be used to initialize
          <c>boot_server</c> with a list of slave IP addresses:</p>
	<p>
          <c>SlaveIP = string() | atom | {integer(),integer(),integer(),integer()}</c>,</p>
        <p>where <c><![CDATA[0 <= integer() <=255]]></c>.</p>
        <p>Examples of <c>SlaveIP</c> in atom, string, and tuple form:</p>
	<p><c>'150.236.16.70', "150,236,16,70", {150,236,16,70}</c>.</p>
        <p>Defaults to <c>[]</c>.</p>
      </item>
      <tag><c>start_disk_log = true | false</c></tag>
      <item>
        <p>Starts the <c>disk_log_server</c> if the parameter is
          <c>true</c> (see <seealso marker="disk_log"><c>disk_log(3)</c></seealso>).
	  This parameter is to be set to <c>true</c> in an embedded system
	  using this service.</p>
        <p>Defaults to <c>false</c>.</p>
      </item>
      <tag><c>start_pg2 = true | false</c></tag>
      <item>
        <marker id="start_pg2"></marker>
        <p>Starts the <c>pg2</c> server (see
	  <seealso marker="pg2"><c>pg2(3)</c></seealso>) if
          the parameter is <c>true</c>. This parameter is to be set to
          <c>true</c> in an embedded system that uses this service.</p>
        <p>Defaults to <c>false</c>.</p>
      </item>
      <tag><c>start_timer = true | false</c></tag>
      <item>
        <p>Starts the <c>timer_server</c> if the parameter is
          <c>true</c> (see <seealso marker="stdlib:timer"><c>timer(3)</c></seealso>).
	  This parameter is to be set to <c>true</c> in an embedded system
	  using this service.</p>
        <p>Defaults to <c>false</c>.</p>
      </item>
      <tag><c>shell_history = enabled | disabled </c></tag>
      <item>
        <p>Specifies whether shell history should be logged to disk
           between usages of <c>erl</c>.</p>
      </item>
      <tag><c>shell_history_drop = [string()]</c></tag>
      <item>
        <p>Specific log lines that should not be persisted. For
           example <c>["q().", "init:stop()."]</c> will allow to
           ignore commands that shut the node down. Defaults to
           <c>[]</c>.</p>
      </item>
      <tag><c>shell_history_file_bytes = integer()</c></tag>
      <item>
        <p>how many bytes the shell should remember. By default, the
           value is set to 512kb, and the minimal value is 50kb.</p>
      </item>
      <tag><c>shell_history_path = string()</c></tag>
      <item>
        <p>Specifies where the shell history files will be stored.
           defaults to the user's cache directory as returned by
           <c>filename:basedir(user_cache, "erlang-history")</c>.</p>
      </item>
      <tag><c>shutdown_func = {Mod, Func}</c></tag>
      <item>
        <p>Where:</p>
        <list type="bulleted">
          <item><c>Mod = atom()</c></item>
          <item><c>Func = atom()</c></item>
        </list>
        <p>Sets a function that <c>application_controller</c> calls
          when it starts to terminate. The function is called as
          <c>Mod:Func(Reason)</c>, where <c>Reason</c> is the terminate
          reason for <c>application_controller</c>, and it must
          return as soon as possible for <c>application_controller</c>
          to terminate properly.</p>
      </item>
      <tag><c>source_search_rules = [DirRule] | [SuffixRule] </c></tag>
      <item>
	<marker id="source_search_rules"></marker>
        <p>Where:</p>
        <list type="bulleted">
          <item><c>DirRule = {ObjDirSuffix,SrcDirSuffix}</c></item>
          <item><c>SuffixRule = {ObjSuffix,SrcSuffix,[DirRule]}</c></item>
          <item><c>ObjDirSuffix = string()</c></item>
          <item><c>SrcDirSuffix = string()</c></item>
          <item><c>ObjSuffix = string()</c></item>
          <item><c>SrcSuffix = string()</c></item>
        </list>
        <p>Specifies a list of rules for use by
	  <seealso marker="stdlib:filelib#find_file/2">
	  <c>filelib:find_file/2</c></seealso>
	  <seealso marker="stdlib:filelib#find_source/2">
	  <c>filelib:find_source/2</c></seealso>
          If this is set to some other value
          than the empty list, it replaces the default rules. Rules can be
          simple pairs of directory suffixes, such as <c>{"ebin",
          "src"}</c>, which are used by <c>filelib:find_file/2</c>, or
          triples specifying separate directory suffix rules depending on
          file name extensions, for example <c>[{".beam", ".erl", [{"ebin",
          "src"}]}</c>, which are used by <c>filelib:find_source/2</c>. Both
          kinds of rules can be mixed in the list.</p>
	  <p>The interpretation of <c>ObjDirSuffix</c> and <c>SrcDirSuffix</c>
	  is as follows: if the end of the directory name where an
	  object is located matches <c>ObjDirSuffix</c>, then the
	  name created by replacing <c>ObjDirSuffix</c> with
	  <c>SrcDirSuffix</c> is expanded by calling
	  <seealso marker="stdlib:filelib#wildcard/1">
	  <c>filelib:wildcard/1</c></seealso>, and the first regular
	  file found among the matches is the source file.
	</p>

      </item>
    </taglist>
  </section>

  <section>
    <title>Deprecated Configuration Parameters</title>
    <p>In Erlang/OTP 21.0, a new API for logging was added. The
      old <c>error_logger</c> event manager, and event handlers
      running on this manager, still work, but they are no longer used
      by default.</p>
    <p>The following application configuration parameters can still be
      set, but they are only used if the corresponding configuration
      parameters for Logger are not set.</p>
    <taglist>
      <tag><c>error_logger</c></tag>
      <item>Replaced by setting the type of the default
      <seealso marker="logger_std_h#type"><c>logger_std_h</c></seealso>
      to the same value. Example:
      <code type="none">
erl -kernel logger '[{handler,default,logger_std_h,#{config=>#{type=>{file,"/tmp/erlang.log"}}}}]'
      </code>
      </item>
      <tag><c>error_logger_format_depth</c></tag>
      <item>Replaced by setting the <seealso marker="logger_formatter#depth"><c>depth</c></seealso>
      parameter of the default handlers formatter. Example:
      <code type="none">
erl -kernel logger '[{handler,default,logger_std_h,#{formatter=>{logger_formatter,#{legacy_header=>true,template=>[{logger_formatter,header},"\n",msg,"\n"],depth=>10}}}]'
      </code>
      </item>
    </taglist>
    <p>See <seealso marker="logger_chapter#compatibility">Backwards
	compatibility with error_logger</seealso> for more
      information.</p>
  </section>

  <section>
    <title>See Also</title>
    <p><seealso marker="app"><c>app(4)</c></seealso>,
      <seealso marker="application"><c>application(3)</c></seealso>,
      <seealso marker="code"><c>code(3)</c></seealso>,
      <seealso marker="disk_log"><c>disk_log(3)</c></seealso>,
      <seealso marker="erl_boot_server"><c>erl_boot_server(3)</c></seealso>,
      <seealso marker="erl_ddll"><c>erl_ddll(3)</c></seealso>,
      <seealso marker="file"><c>file(3)</c></seealso>,
      <seealso marker="global"><c>global(3)</c></seealso>,
      <seealso marker="global_group"><c>global_group(3)</c></seealso>,
      <seealso marker="heart"><c>heart(3)</c></seealso>,
      <seealso marker="inet"><c>inet(3)</c></seealso>,
      <seealso marker="logger"><c>logger(3)</c></seealso>,
      <seealso marker="net_kernel"><c>net_kernel(3)</c></seealso>,
      <seealso marker="os"><c>os(3)</c></seealso>,
      <seealso marker="pg2"><c>pg2(3)</c></seealso>,
      <seealso marker="rpc"><c>rpc(3)</c></seealso>,
      <seealso marker="seq_trace"><c>seq_trace(3)</c></seealso>,
      <seealso marker="user"><c>user(3)</c></seealso>,
      <seealso marker="stdlib:timer"><c>timer(3)</c></seealso></p>
  </section>
</appref>
