public function coerce($units)
{
if ($this->unitless()) {
return new Number($this->dimension, $units);
}
$dimension = $this->dimension;
foreach (static::$unitTable['in'] as $unit => $conv) {
$from = isset($this->units[$unit]) ? $this->units[$unit] : 0;
$to = isset($units[$unit]) ? $units[$unit] : 0;
$factor = pow($conv, $from - $to);
$dimension /= $factor;
}
return new Number($dimension, $units);
}