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 and allows to
implement breadth-first search.
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. Hence, using `concat` to merge the levels, yields
breadth-first search, but different combination function
(e.g. diagonalisation) can be applied too.