b2/: add parsing of substitution rules (WIP)

b2/SUBST

@@ -0,0 +1,25 @@
+REF=R[0-9]* {
+	T=R
+	VAL=.* { R=$$#R }
+	TOL = 5%
+	FN=.*% { TOL=${$#%} }
+	break REF
+//	end break again
+}
+
+/*
+pattern: () * ? .
+subst:  $1 ... $field
+
+substring:
+  $1, $2, ...
+variable:
+  $foo, ...
+with curly braces:
+  ${foo}, ...
+unit conversion:
+  $foo#V
+  ${1#R}
+input variable:
+  $$
+*/