| t = branch(t0, a, t1) ⊦ t = leaf, t .leftmost = a, t .leftmost ∈ t0 |
⊦ branch(t0, a, t1) .leftmost = ((t0 = leaf ⊃ a; t0 .leftmost))
⊦ ((t0 = leaf ⊃ a; t0 .leftmost)) = a, ((t0 = leaf ⊃ a; t0 .leftmost)) ∈ t0
[[CONCRETESIMPRULES : ([CSIMPSEQ : ⊦ leaf ≠ branch(t, a, t0)])]]