code: private String sortedEquivalence(final Equivalence equivalence) {
          final Formula right;
          final Formula left;
          if (this.comparator.compare(equivalence.left(), equivalence.right()) <= 0) {
              right = equivalence.right();
              left = equivalence.left();
          } else {
              right = equivalence.left();
              left = equivalence.right();
          }
          final String leftString = FType.EQUIV.precedence() < left.type().precedence() ? toString(left) : bracket(left);
          final String rightString = FType.EQUIV.precedence() < right.type().precedence() ? toString(right) : bracket(right);
          return String.format("%s%s%s", leftString, equivalence(), rightString);
      }
docs: Returns the string representation of an equivalence.
  
  @param equivalence the equivalence
  @return the string representation