function subtract($polynomialEquation)
 {
     if (!is_a($polynomialEquation, 'Polynomial\\Equation')) {
         $polynomialEquation = new self($polynomialEquation);
     }
     $terms = $polynomialEquation->getTerms();
     foreach ($terms as $term) {
         $this->subtractTerm($term->copy());
     }
     return $this;
 }