reserve L for Boolean non empty RelStr;
reserve a,b,c,d for Element of L;

theorem
  a <= b"\/"c & a"/\"c = Bottom L implies a <= b
proof
  assume a <= b"\/"c & a"/\"c = Bottom L;
  then a"/\"(b"\/"c) = a & a"/\"(b"\/"c) = (a"/\"b)"\/" Bottom L by Th10,
WAYBEL_1:def 3;
  then a"/\"b = a by WAYBEL_1:3;
  hence thesis by YELLOW_0:23;
end;
