theorem Th14:
  x in Lin(A) iff ex l st x = Sum(l)
proof
  thus x in Lin(A) implies ex l st x = Sum(l)
  proof
    assume x in Lin(A);
    then x in the carrier of Lin(A) by STRUCT_0:def 5;
    then x in the set of all Sum(l)  by Def2;
    hence thesis;
  end;
  given k being Linear_Combination of A such that
A1: x = Sum(k);
  x in the set of all Sum(l) by A1;
  then x in the carrier of Lin(A) by Def2;
  hence thesis by STRUCT_0:def 5;
end;
