code: public boolean removeAll(final Collection<?> c) {
          if (N.isNullOrEmpty(c)) {
              return false;
          }
  
          boolean result = false;
  
          for (Object e : c) {
              if (result == false) {
                  result = valueMap.remove(e) != null;
              } else {
                  valueMap.remove(e);
              }
          }
  
          return result;
      }
docs: Removes all of this Multiset's elements that are also contained in the
  specified collection (optional operation).  After this call returns,
  this Multiset will contain no elements in common with the specified
  collection. This method ignores how often any element might appear in
  {@code c}, and only cares whether or not an element appears at all.
  
  @param c
  @return <tt>true</tt> if this set changed as a result of the call
  @see Collection#removeAll(Collection)