/// validates that we have not seen both != and == operators against _field
validates that we have not seen both != and == operators against _field
validates that we have not seen both ! = and == operators against _field
fn validate_operators(&self) -> DataFusionResult<()> {
        if self.seen_eq && self.seen_not_eq {
            Err(DataFusionError::Plan(
                "Unsupported _field predicates: both '=' and '!=' present".into(),
            ))
        } else {
            Ok(())
        }
    }
fn validate_operators ( & self ) -> DataFusionResult < ( ) > { if self . seen_eq && self . seen_not_eq { Err ( DataFusionError :: Plan ( " " . into ( ) , ) ) } else { Ok ( ( ) ) } }
validates that we have not seen both != and == operators against _field
validates that we have not seen both ! = and == operators against _field
&self