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2.3.6.3 XRange Type

The `xrange` type is an immutable sequence which is commonly used for looping. The advantage of the `xrange` type is that an `xrange` object will always take the same amount of memory, no matter the size of the range it represents. There are no consistent performance advantages.

XRange objects have very little behavior: they only support indexing, iteration, and the `len()` function.