function logcdf( x, v ) {
	var x2;
	var p;
	var z;
	if (
		isnan( x ) ||
		isnan( v ) ||
		v <= 0.0
	) {
		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 with degrees of freedom `v` at a value `x`.

@param {number} x - input value
@param {PositiveNumber} v - degrees of freedom
@returns {number} evaluated logCDF

@example
var y = logcdf( 2.0, 0.1 );
// returns ~-0.493

@example
var y = logcdf( 1.0, 2.0 );
// returns ~-0.237

@example
var y = logcdf( -1.0, 4.0 );
// returns ~-1.677

@example
var y = logcdf( NaN, 1.0 );
// returns NaN

@example
var y = logcdf( 0.0, NaN );
// returns NaN

@example
var y = logcdf( 2.0, -1.0 );
// returns NaN