reserve M for finite-degree Matroid,
  A,B,C for Subset of M,
  e,f for Element of M;

theorem Th24:
  A c= B implies Rnk A <= Rnk B
proof
  ex C being independent Subset of M st C c= A & card C = Rnk A by Th18;
  hence thesis by Th17,XBOOLE_1:1;
end;
