[[!comment format=mdwn
 username="joey"
 subject="""comment 1"""
 date="2015-09-30T14:53:02Z"
 content="""
My suggestion in the specific case of the sbuild property would be to
generate the file whenever the property that generates the chroot has to
make a change. The `onChange` combinator accomplishes that.

But if you really want to only run the property if the file doesn't exist,
you can do that by using the `check` combinator. For example:

	check (not <$> doesFileExist f) (createtarball f)
"""]]
