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

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

@example
var v = median( 0.0, 1.0 );
// returns 0.5

@example
var v = median( 4.0, 12.0 );
// returns 8.0

@example
var v = median( -4.0, 4.0 );
// returns 0.0

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

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

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