There's a design pattern for object-oriented programming called a "membrane" that turns out to be just a pair of adjoint monads; an "identity-preserving membrane" is a monad with an inverse. Do either of these have other names in the category theory literature? -- Mike Stay - metaweta@gmail.com http://www.cs.auckland.ac.nz/~mike http://reperiendi.wordpress.com [For admin and other information see: http://www.mta.ca/~cat-dist/ ]