function mean( lambda ) {
	return addon( lambda );
}
Returns the mean of an exponential distribution.

@private
@param {NonNegativeNumber} lambda - rate parameter
@returns {NonNegativeNumber} mean

@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