b2/: finish unit handling and move most of its processing to the match side

The ${foo#unit} syntax didn't really make sense because it created
a large number of potential error conditions on the assignment side
and didn't help with finding compatible fields.

With all this moved to the match side, an invalid syntax simply causes
a mismatch.

b2/SUBST

@@ -1,14 +1,18 @@
 REF=R[0-9]* {
 	T=R
-	VAL=* { R=$$#R }
+	VAL=(#R) { R=$1 }
 	TOL = 5%
-	FN=*% { TOL=${$#%} }
+	FN=*% { TOL=$$ }
 	break REF
 //	end break again
 }
 
 /*
-pattern: () * ? .
+pattern:
+  () |  like in RE
+  * ?   like in glob
+  (#U)  expect a numeric value of unit U (use substring to get canonical value)
+
 subst:  $1 ... $field
 
 substring:
@@ -17,9 +21,15 @@ variable:
   $foo, ...
 with curly braces:
   ${foo}, ...
-unit conversion:
-  $foo#V
-  ${1#R}
 input variable:
   $$
+
+  Caveat:
+
+  Wrong: FN=* { X=$FN }		there is no variable called FN
+  Right: FN=* { X=$$ }		yields the Fn field selected by FN
+
+  Wrong: VAL=(#R) { R=$VAL }	yields literal value
+  Wrong: VAL=(#R) { R=$$ }	yields literal value
+  Right: VAL=(#R) { R=$1 }	yield canonicalized value
 */