reserve i,j,k,n,m for Nat,
  x,y,z,y1,y2 for object, X,Y,D for set,
  p,q for XFinSequence;

theorem Th3: :: from FINSEQ_2:11
x in rng p implies ex
  i being Element of NAT st i in dom p & p.i = x
proof
  assume x in rng p;
  then ex a being object st a in dom p & x = p.a by FUNCT_1:def 3;
  hence thesis;
end;
