A for loop binding's value for one iteration, attached by
loop.cjs -- when set, the reference resolves to this instead of
looking its name up (CASC.md §5.5's body-scoped loop variables).
The name, or an interpolated value that builds it (CASC.md §7.2 "Built names").
The file this reference was written in -- decides whether it may see
a private @*name (CASC.md §5.2). Set when the reference is built.
An unresolved
@{name}(CASC.md §7.1).