theorem Th74:
  <*x1,x2,x3,x4*>=<*x1,x2,x3*>^<*x4*> & <*x1,x2,x3,x4*>=<*x1,x2*>^
<*x3,x4*> & <*x1,x2,x3,x4*>=<*x1*>^<*x2,x3,x4*> & <*x1,x2,x3,x4*>=<*x1*>^<*x2*>
  ^<*x3*>^<*x4*>
proof
  thus <*x1,x2,x3,x4*>=<*x1,x2,x3*>^<*x4*>;
  thus <*x1,x2,x3,x4*>=<*x1,x2*>^<*x3,x4*> by FINSEQ_1:32;
  hence <*x1,x2,x3,x4*>=<*x1*>^(<*x2*>^<*x3,x4*>) by FINSEQ_1:32
    .=<*x1*>^<*x2,x3,x4*> by FINSEQ_1:43;
  thus thesis;
end;
