¹⁴We can represent a term t as a tree, and use strings of numbers to identify positions p in the tree, thus identifying subterms t|_p. For example, for t = f(g(a),h(b)), we have t|_nil = t, t|₁ = g(a), t|₁₁ = a, t|₂ = h(b), and t|₂₁ = b.