Non-Determinism Monad for Tree Search
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This Haskell library provides an implementation of the MonadPlus type
class that represents the search space as a tree whose constructors
represent mzero, return, and mplus.

Such a tree can be used to implement different search strategies,
e.g., by using a queue. It can also be used as a basis for parallel
search strategies that evaluate different parts of the search space
concurrently.