reserve k,n for Nat,
  x,y,z,y1,y2 for object,X,Y for set,
  f,g for Function;
reserve p,q,r,s,t for XFinSequence;
reserve D for set;

theorem Th27:
  p^q = {} implies p={} & q={}
proof
  assume p^q={};
  then 0 = len (p^q)
    .= len p + len q by Def3;
  hence thesis;
end;
