¹⁶We 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 $t{|}_{21}=b$.