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The state of `String` is pretty dire so I have introduced an
intermediary module with turns a `string` into `list ascii`
so that I can reuse all the functions defined on `list` but
not on `string`...

Plus now that `box` is declared as a coercion, we get all the
embeddings Parser -> Box for free!

Until I manage to find a way to get Indexed's [_] to indeed generate
an **implicit** universal quantification, it seems the combinators
will be more usable if the `n` is threaded explicitly. :/