function _log1pNumber(x) {
    if (x >= -1 || config.predictable) {
      return Math.log1p ? Math.log1p(x) : Math.log(x + 1);
    } else {
      // negative value -> complex value computation
      return _log1pComplex(new type.Complex(x, 0));
    }
  }
Calculate the natural logarithm of a `number+1`
@param {number} x
@returns {number | Complex}
@private