⊦ a ∈ bag ↔ count(a, bag) ≠ 0
count(A[m], mkbag(A, i) -- A[n]) = 0, n < # A, m < # A, n < i, m < i, i ≤ # A ⊦ n = m
Used terms: mkbag(A, i) -- A[n], A[m]