-@defun environment &optional frame
-This is a convenience function that returns the value of
-@code{process-environment} after removing unset and duplicate
-variables from it. The optional argument @var{frame}, which should be
-a frame, means return the list of environment variables in effect for
-that frame. The value this function returns is a copy, so changing it
-will have no effect on the environment variables passed to
-subprocesses.
-@end defun
-