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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
"if (':' == ch) {var a = stack.pop()||0; stack.push(a, a)};"
for this case, if there is nothing in the stack, your code is going to push 2 "0" into the stack. Is it so?
But the description mentions": Duplicate value on top of the stack. If there is nothing on top of the stack, push a 0."