S-C. Mu and R. S. Bird. In *Sixth International Conference on Mathematics of Program Construction*, Dagstuhl, Germany, July 2002

[GZipped Postscript]

This paper is devoted to the proof and applications of a theorem giving conditions under which the inverse of a partial function can be expressed as a relational hylomorphism. The theorem is a generalisation of a previous result, due to Bird and de Moor, that gave conditions under which a total function can be expressed a relational fold. The theorem is illustrated with three problems, all dealing with constructing trees with various properties.