src/Maths/Algebra/Matrix.php
public function previousX()
    {
        if ($this->previousXExists()) {
            $this->seekX($this->keyX()-1);
        } else {
            $this->x = null;
        }
        return $this;
    }
public function previousX()
    {
        if ($this->previousXExists()) {
            $this->seekX($this->keyX()-1);
        } else {
            $this->x = null;
        }
        return $this;
    }
Seek to current `x` less 1

@return  $this