multiplicity method
Returns the multiplicity of element in this multiset.
Multiplicity is the number of times an element appears. Returns 0 if the element is not present.
Example:
final ms = MultiSet<int>.fromIterable([1, 1, 2, 2, 2, 3]);
print(ms.multiplicity(2)); // Output: 3
Implementation
int multiplicity(T element) => _elements[element] ?? 0;