function stdev( a, b ) {
	if (
		isnan( a ) ||
		isnan( b ) ||
		!isInteger( a ) ||
		!isInteger( b ) ||
		a > b
	) {
		return NaN;
	}
	return SQRT1O12 * sqrt( ( pow( b-a+1, 2.0 ) - 1.0 ) );
}
Returns the standard deviation of a discrete uniform distribution.

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

@example
var v = stdev( 0, 1 );
// returns ~0.5

@example
var v = stdev( 4, 12 );
// returns ~2.582

@example
var v = stdev( -4, 4 );
// returns ~2.582

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

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

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

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