function bitAndBigNumber (x, y) {
  if ((x.isFinite() && !x.isInteger()) || (y.isFinite() && !y.isInteger())) {
    throw new Error('Integers expected in function bitAnd')
  }

  const BigNumber = x.constructor
  if (x.isNaN() || y.isNaN()) {
    return new BigNumber(NaN)
  }

  if (x.isZero() || y.eq(-1) || x.eq(y)) {
    return x
  }
  if (y.isZero() || x.eq(-1)) {
    return y
  }

  if (!x.isFinite() || !y.isFinite()) {
    if (!x.isFinite() && !y.isFinite()) {
      if (x.isNegative() === y.isNegative()) {
        return x
      }
      return new BigNumber(0)
    }
    if (!x.isFinite()) {
      if (y.isNegative()) {
        return x
      }
      if (x.isNegative()) {
        return new BigNumber(0)
      }
      return y
    }
    if (!y.isFinite()) {
      if (x.isNegative()) {
        return y
      }
      if (y.isNegative()) {
        return new BigNumber(0)
      }
      return x
    }
  }
  return bitwise(x, y, function (a, b) { return a & b })
}
Bitwise and for Bignumbers

Special Cases:
  N &  n =  N
  n &  0 =  0
  n & -1 =  n
  n &  n =  n
  I &  I =  I
 -I & -I = -I
  I & -I =  0
  I &  n =  n
  I & -n =  I
 -I &  n =  0
 -I & -n = -I

@param {BigNumber} x
@param {BigNumber} y
@return {BigNumber} Result of `x` & `y`, is fully precise
@private