reserve Y for non empty set,
  a,b,c,d for Function of Y,BOOLEAN;

theorem
  a 'or' b = 'not' a 'imp' b
proof
  thus 'not' a 'imp' b ='not' 'not' a 'or' b by BVFUNC_4:8
    .=a 'or' b;
end;
