There's an explanation in the earlier article (http://blog.plover.com/prog/haskell/monad-search.html http://blog.plover.com/prog/haskell/monad-search.html), but

There's an explanation in the earlier article (http://blog.plover.com/prog/haskell/monad-search.html http://blog.plover.com/prog/haskell/monad-search.html), but it's because the `bind`/`bd` function takes the solution arrays from its callees, and concatenates the arrays into one big array of solutions, which it then returns to its caller. In the Python code I think this is built into the behavior of the `+=` operator. In the Perl code it's part of the way the `map` call works; it combines what would be `concat . map` in Haskell.