code: func (b *OpenBitSet) ensureCapacity(numBits int64) {
  	b.ensureCapacityWords(bits2words(numBits))
  	// ensureCapacityWords sets numBits to a multiple of 64, but we
  	// want to set it to exactly what the app asked.
  	if numBits > b.numBits {
  		b.numBits = numBits
  	}
  }
docs: /* Ensure that the []int64 is big enough to hold numBits, expanding it if necessary. */