// to check if the value is a number (first an integer, then a float),
// and falling back to string.
func (v *Value) compareOp(op string, a *Value) (res bool, err error) {
	if !isValidComparisonOperator(op) {
		return false, errArithInvalidOperator
	}
	switch op {
	case opIs:
		if a.IsNull() {
			// Missing is null
			return v.IsNull() || v.IsMissing(), nil
		}
		if a.IsMissing() {