New, more flexible, parser context datatype that includes and extends

the previous flags allowForest and allowVarRef.
Added binary operator (work in progress).
Better implementation of +=,-=,*=, and /= (new) operators.

operator-fraction.go

@@ -63,4 +63,5 @@ func evalFraction(ctx ExprContext, opTerm *term) (v any, err error) {
 // init
 func init() {
 	registerTermConstructor(SymVertBar, newFractionTerm)
+	// registerTermConstructor(SymColon, newFractionTerm)
 }