Ronnie Brown asks -
Is it naive to ask that universal algebra theory should also include partial algebras, and in particular groupoids, categories, multiple groupoids, multiple categories (of various kinds)?
It seems that where the methods of universal algebra generalize well to cover partial algebras is in _essentially_ algebraic theories, i.e. those in which there is a hierarchy of operators such that the domain of definition of each is defined by a conjunction of equations involving more primitive operators. This certainly covers categories and groupoids - anyone who knows Phillip Higgins' book can see how smoothly universal algebra works for them. As for multiple groupoids and multiple categories, I can't say because I'm not familiar with them. Steve Vickers. ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++