-construct a filename, so that the lookup cannot access dotfiles or
-files in other directories: values starting with full stops will have
-a colon prepended (making <tt/:./), colons will be doubled, and each
-slash will be replaced with a colon followed by a hyphen <tt>:-</>. A
-parameter value which is the empty string will be replaced with
+construct a filename, so that the lookup cannot access dotfiles,
+backup files, files in other directories and the like: each slash will
+be replaced with a colon followed by a hyphen <tt>:-</>, and all
+characters which are not lowercase alphanumerics, hyphens or
+underscores will have a colon prepended (so that colons are doubled).
+A parameter value which is the empty string will be replaced with