The representable closure (doc/synolon.md §3.1): the fixed bit width of a Synolon type. Computed once, here, for the two consumers that must agree — the machine lint, which rejects a binder, cell, port, or halt answer whose type has no width, and the fold, which lays values out at that width — so the two agree on which types have a width (a type in a position only the fold sizes, such as a definition's codomain or a primitive's instantiation, gets the same diagnostic there).

Widths: Vec n τ is n · |τ| (n nat-closed); Finite n is nbits n; Integer is 128; Proxy is 0; a tuple is the sum of its components; a declared datatype is nbits (#constructors) plus the widest constructor payload (the sum of its field widths at the instantiation); a type variable is 0 (the fold only sees closed types); a recursive datatype, an open width, and a function type have no width.