function init_animal(byref self)
  self.+yell:=@animal_yell;
endfunction
function animal_yell(byref self)
  print(self.text.call(self));
endfunction

function init_cat()
  var cat:=struct;
  init_animal(cat);
  cat.+text:=@cat_text;
  return cat;
endfunction
function cat_text(byref self)
  return "miau";
endfunction

function init_dog()
  var dog:=struct;
  init_animal(dog);
  dog.+text:=@dog_text;
  dog.+angry:=0;
  return dog;
endfunction
function dog_text(byref self)
  if (self.angry)
    return "ggggrrr";
  endif
  return "wuff";
endfunction

var dog:=init_dog();
dog.yell.call(dog);
dog.angry:=1;
dog.yell.call(dog);

var cat:=init_cat();
cat.yell.call(cat);