function kurtosis( a, b ) {
	return addon( a, b );
}
Returns the excess kurtosis of an arcsine distribution.

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

@example
var v = kurtosis( 0.0, 1.0 );
// returns -1.5

@example
var v = kurtosis( 4.0, 12.0 );
// returns -1.5

@example
var v = kurtosis( -4.0, 4.0 );
// returns -1.5

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

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

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