function factory( mu, sigma ) {
	if (
		isnan( mu ) ||
		isnan( sigma ) ||
		sigma < 0.0
	) {
		return constantFunction( NaN );
	}
	if ( sigma === 0.0 ) {
		return degenerate( mu );
	}

	return logcdf;

	/**
	* Evaluates the natural logarithm of the cumulative distribution function (CDF) for a lognormal distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {number} evaluated logcdf
	*
	* @example
	* var y = logcdf( -3.14 );
	* // returns <number>
	*/
	function logcdf( x ) {
		var lx = ( x <= 0.0 ) ? NINF : ln( x );
		return nlogcdf( lx, mu, sigma );
	}
}
Returns a function for evaluating the natural logarithm of the cumulative distribution function (CDF) for a lognormal distribution.

@param {number} mu - mean
@param {NonNegativeNumber} sigma - standard deviation
@returns {Function} logcdf

@example
var logcdf = factory( 10.0, 2.0 );
var y = logcdf( 10.0 );
// returns ~-9.732

y = logcdf( 5.0 );
// returns ~-11.203