{-| An SPDX license, represented in a type that allows logical processing while
    still serializing to a value that Cabal understands.

NB: Cabal doesn’t support the “NOASSERTION” case, so licenses are handled as
    `Optional License`, with `None` (not to be confused with `NONE`)
    representing “NOASSERTION” in the boolean logic.
-}
λ(compound-expression : Type) → < NONE | Expression : compound-expression >