axiom
X(90) Y(90) Z(90) S(1) d;

rules 2
  a= b-F+cFc+F-d&F^d-F+&&cFc+F+b<<;
  b= a&F^cFb^F^d^^-F-d^ -- F^b -- Fc^F^a<<;
  c= -- d^ -- F^b-F+c^F^a&&Fa&F^c+F+b^F^d<<;
  d= -- cFb-F+b -- Fa&F^a&&Fb-F+b -- Fc<<;
end

rules 1
 a=;
 b=;
 c=;
 d=;
end