theorem Th48:
  x is Vector of M iff x in setvect(M)
proof
  thus x is Vector of M implies x in setvect(M);
  thus x in setvect(M) implies x is Vector of M
  proof
    assume x in setvect(M);
    then ex X st x = X & X is Vector of M;
    hence thesis;
  end;
end;
