variables usually uppercase. Otherwise, use another syntax:
&_ => sub_dir_ or TOP_
-&/ => sub/dir/ or nothing
&=_ => sub_dir or TOP
+
+&/ => sub/dir/ or nothing
&=/ => sub/dir or .
&^ => $(top_srcdir)/sub/dir or $(top_srcdir)
&~ => $(abs_top_srcdir)/sub/dir or $(abs_top_srcdir)