function _sqrtNumber(x) {
    if (isNaN(x)) {
      return NaN;
    } else if (x >= 0 || config.predictable) {
      return Math.sqrt(x);
    } else {
      return new type.Complex(x, 0).sqrt();
    }
  }
Calculate sqrt for a number
@param {number} x
@returns {number | Complex} Returns the square root of x
@private