For a given set \(X\), denote by \(G_X\) the set of finite directed trees whose edges are labelled by members of \(X\), with two distinguished vertices. In this note, using techniques of rewriting theory, a new proof is given of the theorem of Munn that the free inverse semigroup on \(X\) is isomorphic to a semigroup defined on the set of so-called birooted word trees on \(X\). The free inverse semigroup emerges as a quotient of \(G_X\) regarded as a certain algebra.