reserve F for Field,
  a,b,c,d,e,f,g,h for Element of F;
reserve x,y for Element of [:the carrier of F,the carrier of F,the carrier of
  F:];
reserve F for Field;
reserve PS for non empty ParStr;
reserve x for set,
  a,b,c,d,e,f,g,h,i,j,k,l for Element of [:the carrier of F,
  the carrier of F,the carrier of F:];

theorem Th4:
  PRs(F) c= [:[:C_3(F),C_3(F):],[:C_3(F),C_3(F):]:]
proof
  let x be object;
  4C_3(F) = [:[:C_3(F),C_3(F):],[:C_3(F),C_3(F):]:];
  hence thesis by Def8;
end;
