```-- | The combinators of this library are all pairs of functions going in
-- opposite directions. These pairs are called /cassettes/, sporting two
-- tracks (the two functions), one of which is read is one direction, the
-- other of which (accessed by flipping the cassette) is read in the opossite
-- direction.
--
-- Here is an example specification for the lambda-calculus:
--
-- > varL = K7 leadout leadin where
-- >   leadout k k' s x = k (\ s _ -> k' s x) s (Var x)
-- >   leadin k k' s t@(Var x)  = k (\ s _ -> k' s t) s x
-- >   leadin k k' s t          = k' s t
-- >
-- > absL = K7 leadout leadin where
-- >   leadout k k' s t' x = k (\ s _ -> k' s t' x) s (Lam x t')
-- >   leadin k k' s t@(Lam x t)  = k (\ s _ _ -> k' s t) s t x
-- >   leadin k k' s t            = k' s t
-- >
-- > appL = K7 leadout leadin where
-- >   leadout k s t2 t1 = k (\ s _ -> k' s t2 t1) s (App t1 t2)
-- >   leadin k k' s t@(App t1 t2)  = k (\ s _ _ -> k' s t) s t2 t1
-- >   leadin k k' s t            = k' s t
-- >
-- > parens p = char '(' <> p <> char ')'
-- >
-- > term :: PP Term
-- > term  =   varL --> ident
-- >       <|> absL --> char '\' <> ident <> term
-- >       <|> appL --> parens (term <> sepSpace <> term)
--
-- From this single specification, we can extract a parser,
--
-- > parse term :: PP Term -> String -> Maybe Term
--
-- and also a pretty printer,
--
-- > pretty term :: PP Term -> Term -> Maybe String
--
-- Specifications are built from primitive and derived combinators, which
-- affect the input string in some way. For each constructor of each datatype,
-- we need to write a /lead/, which is a pair of a construction function and a
-- destruction function. Leads are pure combinators that do not affect the
-- input string. By convention, we suffix their name with "L".
--
-- Internally, the primitive combinators are written in CPS. Leads also need
-- to be written in this style, being primitive. They can, however, be
-- automatically generated for every datatype using some Template Haskell
-- hackery (in a separate package). A number of leads for standard datatypes
-- are defined in the 'Text.Cassette.Lead' module.

module Text.Cassette
( module Text.Cassette.Prim