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

@param string $key
@param string $operator
@param mixed $value
@param string $boolean
@return self