function logcdf( x ) {
		var z = ( x - mu ) / sigma;
		if ( z < -1.0 ) {
			return ln( erfcx( -z * INV_SQRT_TWO ) / 2.0 ) - ( abs2(z) / 2.0 );
		}
		// Case: z >= -1.0:
		return log1p( -erfc( z * INV_SQRT_TWO ) / 2.0 );
	}
Evaluates the natural logarithm of the cumulative distribution function (CDF) for a normal distribution.

@private
@param {number} x - input value
@returns {number} evaluated logcdf

@example
var y = logcdf( -3.14 );
// returns <number>