| depths(Leaf, i) + x = depths(t1, i) + y ⊦ x = y ∧ Leaf = t1 |
⊦ depths(Leaf, i) = i '
⊦ depths(t, i) + x = d ' + y → d ∈ depths(t, i)
i ' + x = depths(t1, i) + y, i ∈ depths(t1, i) ⊦ x = y ∧ Leaf = t1
[[CONCRETESIMPRULES : ([CSIMPSEQ : ⊦ d ∈ x → d ∈ x + y], [CSIMPFORWARD : ⊦ d ' + x = y → d ∈ y], [CSIMPSEQ : ⊦ (x + y) + z = x + y + z])]]