combineValidity

private fn

void combineValidity(const  Bitmap av,  size_t aStart,
       const  Bitmap bv,  size_t bStart,  size_t n,  ulong[]  words)

@safe pure nothrow

Fills words with the conjunction of the validity windows of two chunk positions, one packed bit per slot.

`av`	the left validity (sentinel means no nulls)
`aStart`	the first left slot of the window
`bv`	the right validity (sentinel means no nulls)
`bStart`	the first right slot of the window
`n`	the number of slots
`words`	the destination, `packedWords(n)` words