-VARIABLE should be a string. VALUE is optional; if not provided or is
-`nil', the environment variable VARIABLE will be removed. UNSET
-if non-nil means to remove VARIABLE from the environment.
-SUBSTITUTE-ENV-VARS, if non-nil, means to substitute environment
-variables in VALUE with `substitute-env-vars', where see.
-Value is the new value if VARIABLE, or nil if removed from the
-environment.
+VARIABLE should be a string. VALUE is optional; if not provided or
+nil, the environment variable VARIABLE will be removed.