src/Maths/Algebra/Matrix.php
public function previousY()
    {
        if ($this->previousYExists()) {
            $this->seekY($this->keyY()-1);
        } else {
            $this->y = null;
        }
        return $this;
    }
public function previousY()
    {
        if ($this->previousYExists()) {
            $this->seekY($this->keyY()-1);
        } else {
            $this->y = null;
        }
        return $this;
    }
Seek to current `y` less 1

@return  $this