reserve i,j,k,l for natural Number;
reserve A for set, a,b,x,x1,x2,x3 for object;
reserve D,D9,E for non empty set;
reserve d,d1,d2,d3 for Element of D;
reserve d9,d19,d29,d39 for Element of D9;
reserve p,q,r for FinSequence;

theorem Th8:
  b in rng p implies ex i being Nat st i in dom p & p.i = b
proof
  assume b in rng p;
  then ex a being object st a in dom p & b = p.a by FUNCT_1:def 3;
  hence thesis;
end;
