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.

builtin-import.go

@@ -44,7 +44,7 @@ func doImport(ctx ExprContext, name string, dirList []string, it Iterator) (resu
 			var expr *ast
 			scanner := NewScanner(file, DefaultTranslations())
 			parser := NewParser()
-			if expr, err = parser.parseGeneral(scanner, true, true); err == nil {
+			if expr, err = parser.parseGeneral(scanner, allowMultiExpr|allowVarRef, SymEos); err == nil {
 				result, err = expr.Eval(ctx)
 			}
 			if err != nil {