(* 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};