level-monad: Non-Determinism Monad for Level-Wise Search

Non-Determinism Monad for Level-Wise Search
===========================================

This Haskell library provides an implementation of the MonadPlus type
class that enumerates the levels of the search space using
breadth-first search or iterativ deepening.

A search space is formed by calls to `return` and `mplus` yielding a
search tree with solutions in its leaves. For example, in the monadic
action

    return 1 `mplus` ((return 2 `mplus` return 3) `mplus` return 4)

the result 1 is in the second level, 4 in the third, and the results 2
and 3 are in the forth level. This is apparent from the following
representation of this monadic action:

               `mplus`
             /         \
       return 1          `mplus`
                       /         \
                   `mplus`      return 4
                 /         \
             return 2    return 3

However, the implementation does not build this tree structure as a
data term but constructs its levels directly.

The library provides an operation to get the list of levels from a
non-deterministic computation. The nth element in this list contains
the results of the computation that are found on the nth level of the
computation.