anchor.diff:  get_param_value("path", Name, Req) ->
       Value = cowboy_req:binding(to_binding(Name), Req),
       {ok, Value, Req}.
  -
   
   param_error({enum, Value}, Name) ->
       param_error_(Name, #{error => not_in_enum, data => Value});
  
positive.code:     end.
  
  get_param_value("body", _, Req0) ->
      case cowboy_req:read_body(Req0) of
          {ok, <<>>, Req} -> {ok, <<>>, Req};
          {ok, Body, Req} ->
              try
  
negative.code: param_error({type, Value}, Name) ->
      param_error_(Name, #{error => wrong_type, data => Value});
  param_error({body, Value}, Name) ->
      param_error_(Name, #{error => invalid_body, data => Value});
  param_error({body_too_big, Value}, Name) ->
      param_error_(Name, #{error => body_too_big, data => Value}).
  
  param_error_(Name, Info) ->
      {error, {validation_error, to_binary(Name), Info}}.
  