b2/: meta-variable $ for "previously matched variable"

We need this for iterative processing of FN.

b2/SUBST

@@ -1,5 +1,7 @@
 FN=DNP { ignore }
 
+FN=X(*) { $=$1 }
+
 REF=R[0-9]* {
 	T=R
 	VAL=(#R) { R=$1 }
@@ -23,8 +25,9 @@ variable:
   $foo, ...
 with curly braces:
   ${foo}, ...
-input variable:
+input variable (in pattern):
   $$
+the input variable ($) can also be used as LHS for matches and assignments
 
   Caveat: