function _log1pComplex(x) {
    var xRe1p = x.re + 1;
    return new type.Complex(Math.log(Math.sqrt(xRe1p * xRe1p + x.im * x.im)), Math.atan2(x.im, xRe1p));
  }
Calculate the natural logarithm of a complex number + 1
@param {Complex} x
@returns {Complex}
@private