src/Maths/Algebra/Matrix.php
public function key()
    {
        if ($this->getWalkFlag() & self::WALK_X) {
            return $this->keyX();
        } elseif ($this->getWalkFlag() & self::WALK_Y) {
            return $this->keyY();
        } else {
            return $this->keyXY();
        }
    }
public function key()
    {
        if ($this->getWalkFlag() & self::WALK_X) {
            return $this->keyX();
        } elseif ($this->getWalkFlag() & self::WALK_Y) {
            return $this->keyY();
        } else {
            return $this->keyXY();
        }
    }
Get current index according to matrix's flag value

@return  string|null
@see     self::keyX()
@see     self::keyY()
@see     self::keyXY()