function logpdf( x, v ) {
	return addon( x, v );
}
Evaluates the log probability density function (logPDF) for a Student's t distribution with degrees of freedom `v` at a value `x`.

@private
@param {number} x - input value
@param {number} v - degrees of freedom
@returns {number} evaluated logPDF

@example
var y = logpdf( 0.5, 1.0 );
// returns ~-1.3678734371636097

@example
var y = logpdf( 0.0, 2.0 );
// returns ~-1.0397207708399179

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

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

@example
var y = logpdf( 0.0, -1.0 );
// returns NaN