-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
+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