function entropy( lambda ) {
	if ( isnan( lambda ) || lambda < 0.0 ) {
		return NaN;
	}
	return 1.0 - ln( lambda );
}
Returns the differential entropy of an exponential distribution.

@param {NonNegativeNumber} lambda - rate parameter
@returns {number} differential entropy

@example
var v = entropy( 9.0 );
// returns ~-1.197

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

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

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