public function setFallbackValue( $variableSet, $alwaysUseFallbackValue = false )
	{
		if ( is_null( $this->var->fallbackValue ) ) return false;
		if ( ! $alwaysUseFallbackValue && ! is_null( $this->facts ) ) return false;

		$this->facts = null;

		$vars = $variableSet->getParametersAsVars();

		// Evaluate the fallback value.  Should return an XPath2Item
		$fallbackResult = $this->var->evaluateXPath( $variableSet, $this->var->fallbackValue, $vars );
		$this->yieldedFact = $fallbackResult;
		$this->isFallback = true;

		return true;
	}