proc vRShift(z: var openArray[Digit]; a: openArray[Digit]; m: int; d: int): Digit {.
...raises: [], tags: [], forbids: [].}
Shift digit vector a[0:m] right by d bits, with 0 <= d < digitBits. Put the result in z[0:m], and return the d bits shifted out of the bottom.
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