template newBitVectorStack[B](bits: int, init): untyped = #using defined at
runtime length array
  var blocks = ceilDiv(bits, sizeOf(B)8)
  var bv: BitVecV[B]

  type T = B
  bv.Base = newVLA(B, blocks, init)
  bv.size = bits
  bv

template newVLA(T: typedesc, n: int, init = 0): untyped =