
theorem Th75:
  for H being non empty RelStr st H is Heyting for a,b,c being
  Element of H st a <= b holds (b => c) <= (a => c)
proof
  let H be non empty RelStr;
  assume
A1: H is Heyting;
  let a,b,c be Element of H;
  assume a <= b;
  then
A2: a"/\"(b => c) <= b"/\"(b => c) by A1,Th1;
  b"/\"(b => c) <= c by A1,Lm5;
  then a"/\"(b => c) <= c by A1,A2,ORDERS_2:3;
  hence thesis by A1,Th67;
end;
