function kurtosis( a, b ) {
	var n2;
	if (
		isnan( a ) ||
		isnan( b ) ||
		!isInteger( a ) ||
		!isInteger( b ) ||
		a > b
	) {
		return NaN;
	}
	n2 = pow( b - a + 1.0, 2.0 );
	return -1.2 * ( n2 + 1.0 ) / ( n2 - 1.0 );
}
Returns the excess kurtosis of a discrete uniform distribution.

@param {integer} a - minimum support
@param {integer} b - maximum support
@returns {number} excess kurtosis

@example
var v = kurtosis( 0, 1 );
// returns -2.0

@example
var v = kurtosis( 4, 12 );
// returns ~-1.23

@example
var v = kurtosis( -4, 4 );
// returns ~-1.23

@example
var v = kurtosis( 1, 0.4 );
// returns NaN

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

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

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