function variance( a, b ) {
	if (
		isnan( a ) ||
		isnan( b ) ||
		!isInteger( a ) ||
		!isInteger( b ) ||
		a > b
	) {
		return NaN;
	}
	return ( pow( b-a+1, 2.0 ) - 1.0 ) / 12.0;
}
Returns the variance of a discrete uniform distribution.

@param {integer} a - minimum support
@param {integer} b - maximum support
@returns {PositiveNumber} variance

@example
var v = variance( 0, 1 );
// returns ~0.25

@example
var v = variance( 4, 12 );
// returns ~6.667

@example
var v = variance( -4, 4 );
// returns ~6.667

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

@example
var v = variance( 0.5, 1 );
// returns NaN

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

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