A Logos closure: %Logos.Fn{params, variadic?, body, env, clauses}. env is the lexically-captured defining Logos.Env -- capturing it is the whole point of fn: it lets the closure see the bindings visible at its definition site even after that scope has otherwise returned, which is what makes it a closure rather than a plain function pointer.

Multi-arity support

To support (fn ([a] body1) ([a b] body2)) multi-arity dispatch, this struct has a clauses field: [%{params: [String.t()], variadic?: boolean(), body: [Logos.Form.t()]}], always populated (one entry for an ordinary single-arity fn, several for multi-arity). params/ variadic?/body at the top level always mirror hd(clauses), kept around so single-arity introspection/pattern-matching against the simple %Logos.Fn{params:, variadic?:, body:} shape still works; Logos.Eval's applier always dispatches through clauses, never the mirrored top-level fields.

params is a list of bare parameter name strings (already stripped of the & rest marker read by Logos.Eval's fn handling); when variadic? is true, the last entry of params is the rest parameter, bound to a (possibly empty) list of the trailing args.