src/bigi/lib/bigi.js
function bnpChangeBit(n, op) {
  var r = BigInteger.ONE.shiftLeft(n)
  this.bitwiseTo(r, op, r)
  return r
}
function bnpChangeBit(n, op) {
  var r = BigInteger.ONE.shiftLeft(n)
  this.bitwiseTo(r, op, r)
  return r
}
(protected) this op (1<<n)