anchor.diff:  	{_, X1, Y1, Z1} = relative_to({0, X, Y, Z}, Rotation),
   	{X1, Y1, Z1}.
   
  +% -------------------------------------------------------------------------
  +
   %% @doc Converts from and axis and angle (radians), to a quaternion.
   from_axis_angle({_, _, _} = Axis, Angle) when is_number(Angle) ->
   	from_axis_angle(radians, Axis, Angle).
  
positive.code: %% @doc Converts from body rates (degrees) to a quaternion.
  from_body_rates(degrees, {X, Y, Z}) ->
  	from_body_rates(radians, {deg2rad(X), deg2rad(Y), deg2rad(Z)}). 
  
  %% @doc Converts from a vector of euler angles (radians) to a quaternion.
  from_euler({_, _, _} = Vec) ->
  
negative.code: from_axis_angle(degrees, Axis, Angle) when is_number(Angle) ->
  	DegAngle = deg2rad(Angle),
  	from_axis_angle(radians, {_, _, _} = Axis, DegAngle).
  
  % -------------------------------------------------------------------------
  
  %% @doc Converts from body rates (radians) to a quaternion.
  from_body_rates({_, _, _} = Vec) ->
  