public AggregateExpression GroupConcat(SparqlExpression expression, string separator = " ")
        {
            GroupConcatAggregate aggregate;

            if (separator != " ")
            {
                aggregate = new GroupConcatAggregate(expression.Expression, Constant(separator).Expression, _distinctAggregate);
            }
            else
            {
                aggregate = new GroupConcatAggregate(expression.Expression, _distinctAggregate);
            }

            return(new AggregateExpression(aggregate));
        }
        public AggregateExpression GroupConcat(VariableTerm variable, string separator = " ")
        {
            GroupConcatAggregate aggregate;

            if (separator != " ")
            {
                aggregate = new GroupConcatAggregate(variable, Constant(separator).Expression, _distinctAggregate);
            }
            else
            {
                aggregate = new GroupConcatAggregate(variable, _distinctAggregate);
            }

            return(new AggregateExpression(aggregate));
        }