theorem
  FreeAtoms(X --> M1) = [: X, the carrier of M1 :]
proof
  Union disjoin Carrier(X --> M1)
     = Union disjoin(X --> the carrier of M1) by PRALG_1:16
    .= [: the carrier of M1, X :] by CARD_3:25;
  hence thesis by SYSREL:5;
end;
