function pdf( x, v ) {
	return addon( x, v );
}
Evaluates the probability density function (PDF) 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 PDF

@example
var y = pdf( 2.0, 1.0 );
// returns ~0.0637

@example
var y = pdf( 0.0, 1.0 );
// returns ~0.318

@example
var y = pdf( -1.0, 4.0 );
// returns ~0.215

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

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

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