"Variance information."​
​shared interface Variance of invariant | covariant | contravariant {}
​
"Invariant means that neither subtype nor supertype can be accepted, the
 type has to be exactly that which is declared."​
​shared object invariant satisfies Variance {
    string => "Invariant";
}
​
"Covariant means that subtypes of the given type may be returned."​
​shared object covariant satisfies Variance {
    string => "Covariant";
}
​
"Contravariant means that supertypes of the given type may be accepted."​
​shared object contravariant satisfies Variance {
    string => "Contravariant";
}
​