{"ast_node_type":"other","tokens":[{"end":75,"kind":"keyword","start":0,"text":"Invariant,"}]}
    Invariant,     // T<A> <: T<B> iff B == A -- e.g., type of mutable cell
    Covariant,     // T<A> <: T<B> iff A <: B -- e.g., function return type
    Contravariant, // T<A> <: T<B> iff B <: A -- e.g., function param type