function pdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x < 0.0 ) {
			return 0.0;
		}
		return exp( -x / scale ) / scale;
	}
Evaluates the probability density function (PDF) for an exponential distribution.

@private
@param {number} x - input value
@returns {number} evaluated PDF

@example
var y = pdf( 2.3 );
// returns <number>