| ⊦ count(a, bag) = 0, bag = (bag -- a) ++ a, ∃ bag0. bag = bag0 ++ a |
⊦ bag0 = bag1 ↔ (∀ a. count(a, bag0) = count(a, bag1))
⊦ count(a, bag) = 0, ∀ a0. count(a0, bag) = count(a0, (bag -- a) ++ a), ∃ bag0. bag = bag0 ++ a
Used terms: (bag -- a) ++ a, bag