public function getAdditionalNamespaces( $variableSet, $evaluationResult )
	{
		$namespaces = array();

		foreach ( $this->multiplyBy as $numerator )
		{
			if ( ! isset( $numerator['measure'] ) || ! $numerator['measure'] ) continue;

			// This is an expression that needs evaluating
			$result = $this->evaluateXPath( $variableSet, "{$numerator['measure']}", $evaluationResult['vars'] );
			if ( $result instanceof XPath2Item )
			{
				$result = $result->getTypedValue();
			}
			// Should report an error here
			if ( ! $result instanceof QNameValue ) continue;
			$namespaces[ $result->Prefix ] = $result->NamespaceUri;
		}

		foreach ( $this->divideBy as $denominator )
		{
			if ( ! isset( $denominator['measure'] ) || ! $denominator['measure'] ) continue;

			// This is an expression that needs evaluating
			$result = $this->evaluateXPath( $variableSet, "{$denominator['measure']}", $evaluationResult['vars'] );
			if ( $result instanceof XPath2Item )
			{
				$result = $result->getTypedValue();
			}
			// Should report an error here
			if ( ! $result instanceof QNameValue ) continue;
			$namespaces[ $result->Prefix ] = $result->NamespaceUri;
		}

		return $namespaces;
	}