function y1( x ) {
	return addon( x );
}
Computes the Bessel function of the second kind of order one.

@private
@param {number} x - input value
@returns {number} evaluated Bessel function

@example
var v = y1( 0.0 );
// returns -Infinity

@example
var v = y1( 1.0 );
// returns ~-0.781

@example
var v = y1( -1.0 );
// returns NaN

@example
var v = y1( Infinity );
// returns 0.0

@example
var v = y1( -Infinity );
// returns NaN

@example
var v = y1( NaN );
// returns NaN