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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.
Numbers with a leading zero are (in some programming languages) interpreted as octal (base 8). 17 in the octal number system is 15 in the decimal numer system (base 10). squareDitigts(15) gives 125, since 1² = 1, and 5² = 25.
You can use the hexadecimal system by prefixing the number with 0x, and binary with 0b. At least in JavaScript.
017 = 0xf = 0b1111 = 15.