function mean( k, lambda ) {
	if (
		!isPositiveInteger( k ) ||
		isnan( lambda ) ||
		lambda <= 0.0
	) {
		return NaN;
	}
	return k / lambda;
}
Returns the expected value of an Erlang distribution.

@param {PositiveInteger} k - shape parameter
@param {PositiveNumber} lambda - rate parameter
@returns {NonNegativeNumber} expected value

@example
var v = mean( 1, 1.0 );
// returns 1.0

@example
var v = mean( 4, 12.0 );
// returns ~0.333

@example
var v = mean( 8, 2.0 );
// returns 4.0

@example
var v = mean( 1.5, 2.0 );
// returns NaN

@example
var v = mean( 1, -0.1 );
// returns NaN

@example
var v = mean( -0.1, 1.0 );
// returns NaN

@example
var v = mean( 2, NaN );
// returns NaN

@example
var v = mean( NaN, 2.0 );
// returns NaN