end FUNCTION getCDF() 
FUNCTION: getQuantileFunction( mu, variance )
Returns a quantile function for a normal distribution with mean `mu` and variance `s2`.

@private
@param {Number} mu - distribution mean
@param {Number} variance - distribution variance
@returns {Function} quantile function
function getQuantileFunction( mu, s2 ) {
		var A = mu,
			B = Math.sqrt( s2*2 );
		/**
		* FUNCTION: quantile( x )
		*	Evaluates the quantile function at input value `x`.
		*
		* @private
		* @param {Number} x - input value
		* @returns {Number} evaluated quantile function
		*/
		return function quantile( x ) {
			var C = 2*x - 1;
			return A + B*erfinv( C );
		};
	}