theorem Th11:
  c in -B implies ex g st (for s st s in B holds g.s in s`1 \/ s`2
) & c = [{ g.t1 : g.t1 in t1`2 & t1 in B }, { g.t2 : g.t2 in t2`1 & t2 in B }]
proof
  assume c in -B;
  then
  c in { [{ g.t1 : g.t1 in t1`2 & t1 in B }, { g.t2 : g.t2 in t2`1 & t2 in
  B }] : s in B implies g.s in s`1 \/ s`2 } by XBOOLE_0:def 4;
  then
  ex g st c = [{ g.t1 : g.t1 in t1`2 & t1 in B }, { g.t2 : g.t2 in t2`1 &
  t2 in B }] & for s st s in B holds g.s in s`1 \/ s`2;
  hence thesis;
end;
