code: public function where($key, $operator, $value=null, $boolean=Queryable::AND_)
      {
          return $this->add($this->newExpression($key, $operator, $value, $boolean));
      }
docs: {@inheritdoc} Adds a new expression and returns itself
  
  @param string $key
  @param string $operator
  @param mixed $value
  @param string $boolean
  @return self