unreadable?

do_set_trace_flags(Tracer,Args,How) ->
    Fun=fun({Proc,Flags}) ->
		case check_traceflag_pidspec(Proc) of
		    {ok,Proc2} ->            % Reg-names converted.
			case check_flags(Flags) of
			    Flags2 when is_list(Flags2) -> % No error!
				case (catch
					 case How of
					     true ->
						 erlang:trace(Proc2,
							      true,
							      [{tracer,Tracer}|Flags2]);
					     false -> % No tracer of turning off.
						 erlang:trace(Proc2,
							      false,
							      Flags2)
					 end) of
				    N when is_integer(N) ->
					N;
				    {'EXIT',Reason} ->
					if
					    is_pid(Proc2) ->
						0;    % Proc2 not alive or not at this node!
					    true ->   % Otherwise, just error!
						{error,
						 {bad_trace_args,
						  [Reason,Proc2,How,Flags2,Tracer]}}
					end
				end;
			    FlagError ->
				FlagError
			end;
		    false ->                 % Skip it.
			0;                   % Indicate that zero processes matched.
		    {error,Reason} ->        % Bad process specification.
			{error,{bad_process,[Reason,Proc]}}
		end;
	   (Faulty) ->
		{error,{bad_process,Faulty}}
	end,
    {ok,lists:map(Fun,Args)}.