-#### If the contents are same, the second file is removed. And the
-#### first file is removed unless there's an optional argument with
-#### a value of 'keep first'.
-#### If different, global variable "failed" is incremented.
-#### If there's an optional 'ignore space' argument, spacing differences
-#### will not be considered signficant, emulating GNU diff -w. It
-#### is assumed that the first file has already been run through
-#### prepare_space.
+#### If the contents are same, the second file is removed. If different,
+#### global variable "failed" is incremented.
+#### Optional arguments:
+#### 'keep first' -- first file is removed unless this is present.
+#### 'ignore space' -- spacing differences will not be considered
+#### signficant, emulating GNU diff -w. It is assumed that the
+#### first file has already been run through prepare_space.
+#### ':' <test name> -- will print '<test name>' in the failure message,
+#### to make it easier to tell which of multiple tests has failed.