reserve Y for non empty set;

theorem Th8:
  for a,b being Function of Y,BOOLEAN holds a 'imp' b = 'not' a 'or' b
proof
  let a,b be Function of Y,BOOLEAN;
  let x be Element of Y;
  thus (a 'imp' b).x = a.x => b.x by BVFUNC_1:def 8
      .= 'not' a.x 'or' b.x
      .= ('not' a).x 'or' b.x by MARGREL1:def 19
      .= ('not' a 'or' b).x by BVFUNC_1:def 4;
end;
