public function sub(Distance $b)
    {
        return Distance::m(bcsub($this->m, $b->m), static::$s);
    }
public function sub(Distance $b)
    {
        return Distance::m(bcsub($this->m, $b->m), static::$s);
    }
public function sub ( Distance $ b ) { return Distance :: m ( bcsub ( $ this -> m , $ b -> m ) , static :: $ s ) ; }
Subtracts another distance from this instance and returns a new instance
with the difference.

@param Distance $b Distance to subtract

@return static Difference of the two distances
Subtracts another distance from this instance and returns a new instance with the difference .