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

theorem
  a \ b <= c implies a <= b"\/"c
proof
A1: a <= a"\/" b by YELLOW_0:22;
  assume a \ b <= c;
  then
A2: (a"/\"'not' b)"\/"b <= c"\/"b by Th7;
  (a"/\"'not' b)"\/"b = (b"\/"a) "/\" (b"\/"'not' b) by Th17
    .= (b"\/"a) "/\" Top L by Th34
    .= a"\/"b by WAYBEL_1:4;
  hence thesis by A2,A1,YELLOW_0:def 2;
end;
