code: function cdf( x, sigma ) {
  	if ( x < 0 ) {
  		return 0;
  	}
  	var s2 = pow( sigma, 2 );
  	return -expm1( - pow( x, 2) / ( 2 * s2 ) );
  }
docs: CDF // 
  FUNCTION: cdf( x, sigma )
  Evaluates the cumulative distribution function (CDF) for a Rayleigh distribution with scale parameter `sigma` at a value `x`.
  
  @param {Number} x - input value
  @param {Number} sigma - scale parameter
  @returns {Number} evaluated CDF