parser: token '()' returns error

parser.go

@@ -455,15 +455,6 @@ func (parser *parser) parseGeneral(scanner *scanner, ctx parserContext, termSymb
 		//fmt.Println("Token:", tk)
 		if firstToken {
 			changePrefix(tk)
-			// if tk.Sym == SymMinus {
-			// 	tk.Sym = SymChangeSign
-			// } else if tk.Sym == SymPlus {
-			// 	tk.Sym = SymUnchangeSign
-			// } else if tk.IsSymbol(SymStar) {
-			// 	tk.SetSymbol(SymDereference)
-			// } else if tk.IsSymbol(SymExclamation) {
-			// 	tk.SetSymbol(SymNot)
-			// }
 			firstToken = false
 		}
 
@@ -471,12 +462,13 @@ func (parser *parser) parseGeneral(scanner *scanner, ctx parserContext, termSymb
 		case SymOpenRound:
 			var subTree *ast
 			if subTree, err = parser.parseGeneral(scanner, ctx, SymClosedRound); err == nil {
-				exprTerm := newExprTerm(subTree.root)
-				err = tree.addTerm(exprTerm)
-				currentTerm = exprTerm
-				// subTree.root.priority = priValue
-				// err = tree.addTerm(newExprTerm(subTree.root))
-				// currentTerm = subTree.root
+				if subTree.root == nil {
+					err = tk.ErrorExpectedGotString("expression", "()")
+				} else {
+					exprTerm := newExprTerm(subTree.root)
+					err = tree.addTerm(exprTerm)
+					currentTerm = exprTerm
+				}
 			}
 		case SymFuncCall:
 			var funcCallTerm *term