anchor.diff:  % -------------------------------------------------------------------------
   
   % external api
  --export([simulate/2]).
  +-export([simulate/1, simulate/2, default_physical/0, to_proplist/1, from_proplist/1, update_from_proplist/2, get_prop/2]).
   
   % -------------------------------------------------------------------------
   
  
positive.code: 
  %% ------------------------------------------------------------------------
  %% External API
  %% ------------------------------------------------------------------------
  
  -spec evaluate(TimeDelta, Velocity, Force, Spin, Torque, State) -> {Velocity, Force, Spin, Torque, State} when
  
negative.code: 
  %% ------------------------------------------------------------------------
  
  simulate_internal(TimeDelta, State) ->
  	#physical{
  		position = Position,
  