I believe at one time even though merged had not been defined, the setting of merged[1, 2] would succeed, without creating merged. This led to an ugly situation:
1. 'Say merged[1, 2]' produces "defined".
2. The same element is invisible to a 'Loop Over', because without merged there is no clue that it exists.
If I remember correctly, the (silent) setting of merged[1} was added to avoid this ugly situation. Personally, I thought it something the programmer probably didn't intend and really needed to know about; and deserved an Exception.