OP solved it, closing

It works because self.area of Rectangle is defined within the parent class Shape

Could you maybe hint on how is this magic with * called? I am trying to google it but cannot find. Update: found it (http://docs.python.org/dev/reference/expressions.html#calls)

You have to use BigInt since the permutations can get really big.

Because problem needs answer to be rounded to the next integer. This trick works like calling ::ceil.
If the extra have a decimal point value greater than or equal to 0.001 this addition will push the extra to next integer and explicit conversion to long long will get rid of remaining decimal points.

('555', 2) the same as ('555555', 1), where you can remove or leave every digit (2^6) minus case, when you deleted all digit (it is not allowed to delete all the digits): 2^6-1

These are not kata issues.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/shift
Shift always removes the first item

Not a kata issue, your code is wrong and it fails when called more than once. a single call works. Now figure out how to fix it ;)

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