theorem Th23:
  |- f^<*p '&' q*> implies |- f^<*q*>
proof
A1: p '&' q = Suc(f^<*p '&' q*>) by CALCUL_1:5;
  assume |- f^<*p '&' q*>;
  then |- Ant(f^<*p '&' q*>)^<*q*> by A1,CALCUL_1:41;
  hence thesis by CALCUL_1:5;
end;
