Many generic programs require information about the recursive positions
of a datatype. Examples include the generic fold, generic rewriting or
the Zipper data structure. Several generic programming systems allow to
write such functions by viewing datatypes as fixed points of a pattern
functor. Traditionally, this view has been limited to so-called regular
datatypes such as lists and binary trees. In particular, families of
mutually recursive datatypes have been excluded.

With the multirec library, we provide a mechanism to talk about fixed
points of families of datatypes that may be mutually recursive. On top
of this representations, generic functions such as the fold or the Zipper
can then be defined.

We expect that the library will be especially interesting for compiler
writers, because ASTs are typically families of mutually recursive datatypes,
and with multirec it becomes easy to write generic functions on ASTs.

The library is based on ideas described in the paper:

Alexey Rodriguez, Stefan Holdermans, Andres Löh, Johan Jeuring.
*Generic programming with fixed points for mutually recursive datatypes*.
ICFP 2009.