function logcdf( x ) {
		var x2;
		var p;
		var z;
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x === 0.0 ) {
			return LN_HALF;
		}
		x2 = pow( x, 2.0 );
		if ( v > 2.0*x2 ) {
			z = x2 / ( v + x2 );
			p = betainc( z, 0.5, v/2.0, true, true ) / 2.0;
		} else {
			z = v / ( v + x2 );
			p = betainc( z, v/2.0, 0.5, true, false ) / 2.0;
		}
		return ( x > 0.0 ) ? log1p( -p ) : ln( p );
	}
Evaluates the natural logarithm of the cumulative distribution function (CDF) for a Student's t distribution.

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

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