reserve
  a for natural Number,
  k,l,m,n,k1,b,c,i for Nat,
  x,y,z,y1,y2 for object,
  X,Y for set,
  f,g for Function;
reserve p,q,r,s,t for FinSequence;
reserve D for set;

theorem Th35:
  p^q = {} implies p = {} & q = {}
proof
  assume p^q={};
  then 0 = len (p^q) .= len p + len q by Th22;
  hence thesis;
end;
