function logcdf( x ) {
		var lx = ( x <= 0.0 ) ? NINF : ln( x );
		return nlogcdf( lx, mu, sigma );
	}
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>