
theorem Th20:
  for S1,S2 being non empty ManySortedSign, v being Vertex of S1
  holds v in the carrier of S1+*S2 & v in the carrier of S2+*S1
proof
  let S1,S2 be non empty ManySortedSign;
  let v be Vertex of S1;
  the carrier of (S1+*S2) = (the carrier of S1) \/ the carrier of S2 & the
carrier of (S2+*S1) = (the carrier of S2) \/ the carrier of S1 by
CIRCCOMB:def 2;
  hence thesis by XBOOLE_0:def 3;
end;
