anchor.diff:  
   	C1 = (f1(A,B,Psi)-i12(A,B)*A_der*A_der-i14(A,B)*B_der*B_der-i15(A,B)*A_der*B_der)*Tau*Tau
   		+ i11(A,B)*(A+A_der*Tau) + i13(A,B)*(B+B_der*Tau),
  -	C2 = (f2(A,B) - i22(A,B)*A_der*A_der - i24(A,B)*B_der*B_der)*Tau*Tau 
  +	C2 = (f2(A,B) - i22(A,B)*A_der*A_der - i24(A,B)*B_der*B_der - i25(A)*A_der*B_der)*Tau*Tau 
   		+ i21(A)*(A+A_der*Tau) + i23(A)*(B+B_der*Tau),
   	%io:format("I1=~p~n",[?I1]),
   	%io:format("I11=~p~n",[E12]),
  
positive.code: -define(Foot,0.05).
  -define(Eta, math:atan(?L/?Foot)).
  -define(Rh, math:sqrt(?L*?L + ?Foot*?Foot)).
  -define(Eta0, 0.5).
  -define(R0, 0.04). % distance from tip of foot to the leg's CoM
  
  %% springs params
  
negative.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  + i25*A'B' = f2(A,B)
  %
  i11(_A,_B) -> ?I1 - ?K0 + ?M1*?D*?D.
  i12(_A,_B) -> 0.
  