Beef up core variance.

[sigilante]

~master-morzod:

"a function fits an interface if a) its argument type is equal-to or less specific and b) its result type is equal-to or more specific than those of the interface"

no, i'm describing %iron / |~` / ^| / $-
%lead / ^? is bivariant
%lead / ^? is bivariant (wrt the payload)
(in a completely trivial way, since the payload is not observable from outside the core)

%lead and %gold (invariant) pertain to the entire payload, %iron and %zinc pertain to the sample (head of the payload), and both have an effectively %lead context (tail of the payload). additionally, %iron lets you write but not read the sample, while %zinc lets you read but not write the sample

and yes, all 4 are covariant (ie, follow normal nesting rules) with regard to the products of their arms