public override ParserRuleContext VisitExpression([NotNull] StructuredTextParser.ExpressionContext context)
            {
                var result = Visit(context.lhs);

                _contextStack.Push(result);
                for (int i = 1; i < context.ChildCount; i += 2)
                {
                    //TODO: 実装
                    throw new NotImplementedException();
                }
                _contextStack.Pop();
                return(result);
            }
 /// <summary>
 /// Visit a parse tree produced by <see cref="StructuredTextParser.expression"/>.
 /// <para>
 /// The default implementation returns the result of calling <see cref="AbstractParseTreeVisitor{Result}.VisitChildren(IRuleNode)"/>
 /// on <paramref name="context"/>.
 /// </para>
 /// </summary>
 /// <param name="context">The parse tree.</param>
 /// <return>The visitor result.</return>
 public virtual Result VisitExpression([NotNull] StructuredTextParser.ExpressionContext context)
 {
     return(VisitChildren(context));
 }