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 );