/*
 * call-seq:
 *    res.getisnull(tuple_position, field_position) -> boolean
 *
 * Returns +true+ if the specified value is +nil+; +false+ otherwise.
 */
static VALUE
pgresult_getisnull(VALUE self, VALUE tup_num, VALUE field_num)
{
    PGresult *result;
    int i = NUM2INT(tup_num);
    int j = NUM2INT(field_num);

    result = get_pgresult(self);
    if (i < 0 || i >= PQntuples(result)) {
        rb_raise(rb_eArgError,"invalid tuple number %d", i);
    }
    if (j < 0 || j >= PQnfields(result)) {
        rb_raise(rb_eArgError,"invalid field number %d", j);
    }
    return PQgetisnull(result, i, j) ? Qtrue : Qfalse;
}