private sealed class ReplaceVisitor : ExpressionVisitor
    {
      public ReplaceVisitor(Expression what, Expression to) {
        if(what == null) {
          throw new ArgumentNullException("what");
        } else if(to == null) {
          throw new ArgumentNullException("to");
        }//if


        What = what;
        To = to;
      }

      public Expression What { get; private set; }
      public Expression To { get; private set; }

      public override Expression Visit(Expression node) {
        if(node == What) {
          return To;
        }//if

        return base.Visit(node);
      }
    }