add a note about undef values
add a note about undef values

leavesbnw/picrin,leavesbnw/picrin,picrin-scheme/picrin,picrin-scheme/picrin,dcurrie/picrin,koba-e964/picrin,ktakashi/picrin,omasanori/picrin,dcurrie/picrin,omasanori/picrin,ktakashi/picrin,leavesbnw/picrin,ktakashi/picrin,koba-e964/picrin,koba-e964/picrin