-# The optional sevenths argument is a list of OS subdirectory names.
-# The format is the same as of the second argument.
-# The difference is that second argument describes multilib directories
-# in GCC conventions, while this one the OS multilib convention.
+# The optional seventh argument is a list of OS subdirectory names.
+# The format is either the same as of the second argument, or a set of
+# mappings. When it is the same as the second argument, it describes
+# the multilib directories using OS conventions, rather than GCC
+# conventions. When it is a set of mappings of the form gccdir=osdir,
+# the left side gives the GCC convention and the right gives the
+# equivalent OS defined location. If the osdir part begins with a !,
+# the os directory names are used exclusively. Use the mapping when
+# there is no one-to-one equivalence between GCC levels and the OS.