A value that is either present (``just v``) or absent (``none``).
Models Python's ``None`` sentinel, partial functions, and any operation
that may not produce a result. ``Maybe a`` is a polymorphic inductive
type so the same library serves ``Maybe Int``, ``Maybe String``, etc.
For each type in this module: inductive types list their declared constructors; opaque types list any local function whose name starts with mk and which returns a value of that type.