anchor.diff:  -define(Foot,0.05).
   -define(Eta, math:atan(?L/?Foot)).
   -define(Rh, math:sqrt(?L*?L + ?Foot*?Foot)).
  --define(Eta0, 0.5).
  +-define(Eta0, 0.45).
   -define(R0, 0.04). % distance from tip of foot to the leg's CoM
   
   %% springs params
  
positive.code: %
  % I11*A'' + i12*A'^2 + i13*B'' + i14*B'^2 + i15*A'*B' = f1(A,B,Psi)
  %
  % I21*A'' + i22*A'^2 +i23*B'' + i24*B'^2  = f2(A,B)
  %
  i11(_A,_B) -> ?I1 - ?K0 + ?M1*?D*?D.
  i12(_A,_B) -> 0.
  
negative.code: -define(Dz, 0.03). % initial strech of the springs
  
  -define(I1, 1.00e-3). % Inertial moment of the body arounf CoM
  -define(I2, 1.25e4). % Inertial moment of the leg around tip of foot
  -define(K0, 0.01). % The coefficient of friction between the body and the leg
  -define(D, 0.03). % The distance between body CoM and hip
  -define(G, 9.81).
  
  
  init() -> init(1.4, 0.27).
  
  init(Alpha, Beta) -> {Alpha, 0.0, Beta, 0.0, 0.0}.
  
  