code: func (b *OpenBitSet) ensureCapacityWords(numWords int) {
  	if len(b.bits) < numWords {
  		arr := make([]int64, numWords)
  		copy(arr, b.bits)
  		b.bits = arr
  	}
  	b.wlen = numWords
  	if n := int64(numWords) << 6; n > b.numBits {
  		b.numBits = n
  	}
  }
docs: /* Expand the []int64 with the size given as a number of words (64 bits long). */