In case anyone thinks that's an academic point, this is exactly the reason who so many dynamic languages can't tell the difference between 'key is not in hashta

In case anyone thinks that's an academic point, this is exactly the reason who so many dynamic languages can't tell the difference between 'key is not in hashtable' and 'key is in hashtable with value null'. I constantly get bitten by this and similar confusions when writing javascript or clojure.