function pdf( x, alpha, beta ) {
	return addon( x, alpha, beta );
}
Evaluates the probability density function (PDF) for a beta distribution with first shape parameter `alpha` and second shape parameter `beta` at a value `x`.

@private
@param {number} x - input value
@param {PositiveNumber} alpha - first shape parameter
@param {PositiveNumber} beta - second shape parameter
@returns {number} evaluated PDF

@example
var y = pdf( 0.5, 1.0, 1.0 );
// returns 1.0

@example
var y = pdf( 0.5, 2.0, 4.0 );
// returns 1.25

@example
var y = pdf( 0.2, 2.0, 2.0 );
// returns ~0.96

@example
var y = pdf( 0.8, 4.0, 4.0 );
// returns ~0.573

@example
var y = pdf( -0.5, 4.0, 2.0 );
// returns 0.0

@example
var y = pdf( 1.5, 4.0, 2.0 );
// returns 0.0

@example
var y = pdf( 0.5, -1.0, 0.5 );
// returns NaN

@example
var y = pdf( 0.5, 0.5, -1.0 );
// returns NaN

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

@example
var y = pdf( 0.5, NaN, 1.0 );
// returns NaN

@example
var y = pdf( 0.5, 1.0, NaN );
// returns NaN