(* simple variables *)
f := {[x] => x};
f := {[_] => x};

(* literals *)
f := {["foo"] => "!"};
f := {[Bar] => "?"};
f := {[55] => "~"};

(* tuples *)
f := {[<>] => x};
f := {[<x>] => x};
f := {[<x,y>] => x};
f := {[<x,y,z>] => x};