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

theorem
  a <= b & c <= d implies a\d <= b\c
proof
  assume that
A1: a <= b and
A2: c <= d;
  'not' d <= 'not' c by A2,Th37;
  then
A3: a"/\"'not' d <= a"/\" 'not' c by Th6;
  a"/\"'not' c <= b"/\" 'not' c by A1,Th6;
  hence thesis by A3,YELLOW_0:def 2;
end;
