internal override Expression VisitUnary(UnaryExpression u)
        {
            switch (u.NodeType)
            {
            case ExpressionType.Not:
            {
                NativeQuery.not(_coreQueryHandle);
                this.Visit(u.Operand);       // recurse into richer expression, expect to VisitCombination
            }
            break;

            default:
                throw new NotSupportedException($"The unary operator '{u.NodeType}' is not supported");
            }
            return(u);
        }