public function op_minus($other)
{
if ($other instanceof SassColour) {
return $other->op_minus($this);
} elseif (!$other instanceof SassNumber) {
throw new SassNumberException('Number must be a number', SassScriptParser::$context->node);
} else {
$other = $this->convert($other);
return new SassNumber($this->value - $other->value . $this->units);
}
}