function xlog1py( x, y ) {
	if ( x === 0.0 && !isnan( y ) ) {
		return 0.0;
	}
	return x * log1p( y );
}
Computes `x * ln(y+1)` so that the result is `0` if `x = 0`.

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

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

@example
var out = xlog1py( 1.5, 5.9 );
// returns ~2.897

@example
var out = xlog1py( 0.9, 1.0 );
// returns ~0.624

@example
var out = xlog1py( 1.0, 0.0 );
// returns 0.0

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

@example
var out = xlog1py( 1.5, NaN );
// returns NaN

@example
var out = xlog1py( 0.0, NaN );
// returns NaN

@example
var out = xlog1py( NaN, 2.3 );
// returns NaN