Etude 2-2 introduce default values and in Etude 3-1 they disappeared

[thilko]

... if I continue to work with default values I am required to introduce a function head since I use claues with default values:

def area(:rectangle, length \\ 1, width \\ 1) do
    length * width
  end

   == Compilation error on file lib/geom.ex ==
   ** (CompileError) lib/geom.ex:15: def area/3 has default values and multiple clauses, define a function    head with the defaults
       lib/geom.ex:15: (module)
       (stdlib) erl_eval.erl:669: :erl_eval.do_apply/6

I have trouble with introducing the function head since this does not work:

def area(geom \\ :rectangle, length \\ 1, width \\ 1)