Consider adding a Counter Node for counting the number of elements in a disjoint_list

[elham-azd]

Assuming we have the following variable model.disjoint_lists(primary_set_size, num_disjoint_lists), it would be great to have a Counter Node that counts the number of elements in each disjoint_list.

[arcondello]

So something like

model = Model()
s = model.set(5)
length = s.len()

model.states.resize(1)

with model.lock():
    assert s.state().size == length.state()

if I understand correctly?