+-- | The strict state-transformer monad.
+-- A computation of type @'ST' s a@ transforms an internal state indexed
+-- by @s@, and returns a value of type @a@.
+-- The @s@ parameter is either
+--
+-- * an uninstantiated type variable (inside invocations of 'runST'), or
+--
+-- * 'RealWorld' (inside invocations of 'Control.Monad.ST.stToIO').
+--
+-- It serves to keep the internal states of different invocations
+-- of 'runST' separate from each other and from invocations of
+-- 'Control.Monad.ST.stToIO'.
+--
+-- The '>>=' and '>>' operations are strict in the state (though not in
+-- values stored in the state). For example,
+--
+-- @'runST' (writeSTRef _|_ v >>= f) = _|_@