-&,= &,_ &,/ &,CAPS &,lc prefixes $(top_srcdir)/subdir
-&;= &;_ &;/ &;CAPS &;lc prefixes $(abs_top_srcdir)/subdir
+&_ => subdir_ or TOP_
+&/ => subdir/ or nothing
+&=_ => subdir or TOP
+&=/ => subdir or .
+&^ => $(top_srcdir)/subdir or $(top_srcdir)
+&~ => $(abs_top_srcdir)/subdir or $(abs_top_srcdir)