function pdf( x, lambda ) {
	return addon( x, lambda );
}
Evaluates the probability density function (PDF) for an exponential distribution with rate parameter `lambda` at a value `x`.

@private
@param {number} x - input value
@param {PositiveNumber} lambda - rate parameter
@returns {number} evaluated PDF

@example
var y = pdf( 0.3, 4.0 );
// returns ~1.205

@example
var y = pdf( 2.0, 0.7 );
// returns ~0.173

@example
var y = pdf( -1.0, 0.5 );
// returns 0.0

@example
var y = pdf( 0, NaN );
// returns NaN

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

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