function entropy( lambda ) {
	var gen;
	var out;
	if ( isnan( lambda ) || lambda < 0.0 ) {
		return NaN;
	}
	if ( lambda === 0.0 ) {
		return 0.0;
	}
	gen = seriesClosure( lambda );
	out = lambda * ( 1.0-ln(lambda) );
	out += exp( -lambda ) * sumSeries( gen );
	return out;
}
Returns the entropy of a Poisson distribution.

@param {NonNegativeNumber} lambda - mean parameter
@returns {PositiveNumber} entropy

@example
var v = entropy( 9.0 );
// returns ~2.508

@example
var v = entropy( 1.0 );
// returns ~1.305

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

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