code: function _putDelta(s, k, t, v, r)
  {
    var delta = _callDelta(s, k, t, v, r) - 1;
    return (delta == -1 && k == s) ? 0 : delta;
  }
docs: Calculates the delta of a put option.
  
  @private
  @param {Number} s Current price of the underlying
  @param {Number} k Strike price
  @param {Number} t Time to experiation in years
  @param {Number} v Volatility as a decimal
  @param {Number} r Anual risk-free interest rate as a decimal
  @returns {Number} The delta of the put option