function _csch(x) {
  // consider values close to zero (+/-)
  if (x === 0) {
    return Number.POSITIVE_INFINITY;
  } else {
    return Math.abs(2 / (Math.exp(x) - Math.exp(-x))) * sign(x);
  }
}
Calculate the hyperbolic cosecant of a number
@param {number} x
@returns {number}
@private