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.