Tuple type (hm-con "*" TYPES); list type (hm-con "list" (TYPE)).
ocaml-infer-tuple threads substitution through each item left-to-right.
ocaml-infer-list unifies all items with a fresh 'a (giving 'a list for
empty []).
Pretty-printer renders 'Int * Int' for tuples and 'Int list' for lists,
matching standard OCaml notation.
Examples:
fun x y -> (x, y) : 'a -> 'b -> 'a * 'b
fun x -> [x; x] : 'a -> 'a list
[] : 'a list
giles
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