anchor.diff:  list_to_quat([W, X, Y, Z]) ->
   	{W, X, Y, Z}.
   
  -% -------------------------------------------------------------------------
  +% ---------------------------------------------------------------------------------------------------------------------
   
   %% @doc Adds the two quaternions together.
   add({W1, X1, Y1, Z1}, {W2, X2, Y2, Z2}) ->
  
positive.code: inverse({_, _, _, _} = Quat) ->
  	divide(conjugate(Quat), norm(Quat)).
  
  % -------------------------------------------------------------------------
  
  %% @doc
  reciprocal({_, _, _, _} = Quat) ->
  
negative.code: 	DegAngle = deg2rad(Angle),
  	from_axis_angle(radians, {_, _, _} = Axis, DegAngle).
  
  % ---------------------------------------------------------------------------------------------------------------------
  
  %% @doc Converts from body rates (radians) to a quaternion.
  from_body_rates({_, _, _} = Vec) ->
  