Type Alias Settle 

pub type Settle<'a> = dyn FnMut(&mut Value, &[Member], &[Effect]) + 'a;

The settlement of bare $new.n targets (3.2): given the effect JSON, the members holding a symbol and the concrete effects before it, rewrite each symbol’s kind to the one the symbol table records — the kind plan over that prefix allocated to $new.n (new_ids[n].kind).