function logcdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		return ln( ( ONE_OVER_PI * atan2( x-x0, gamma ) ) + 0.5 );
	}
Evaluates the  natural logarithm of the cumulative distribution function (logCDF) for a Cauchy distribution.

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

@example
var y = logcdf( 2.0 );
// returns <number>