function xlog1py( x, y ) {
	return addon( x, y );
}
Computes `x * ln(y+1)` so that the result is `0` if `x = 0`.

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

@example
var v = xlog1py( 3.0, 2.0 );
// returns ~3.296

@example
var v = xlog1py( 0.0, -2.0 );
// returns 0.0