function mean( lambda ) {
	if ( isnan( lambda ) || lambda < 0.0 ) {
		return NaN;
	}
	return 1.0 / lambda;
}
Returns the expected value of an exponential distribution.

@param {NonNegativeNumber} lambda - rate parameter
@returns {NonNegativeNumber} expected value

@example
var v = mean( 9.0 );
// returns ~0.111

@example
var v = mean( 1.0 );
// returns 1.0

@example
var v = mean( -0.2 );
// returns NaN

@example
var v = mean( NaN );
// returns NaN