The trustee and creditors who dispute a debtor's claimed exemptions may file objections to the list of exemptions filed by the debtor with the schedules of assets and liabilities. BR 4003(b). The objections must be filed within thirty days after the conclusion of the creditors' meeting or within such longer period of time the court may allow. The court may hold a hearing to decide whether or not the claimed exemptions should be allowed and in any such hearing the objecting party has the burden of proving the exemptions are not properly claimed. BR 4003(c). It may happen that the dispute over exemptions turns on the outcome of an attempt by the debtor to avoid a lien on property claimed as exempt since property subject to a valid lien is not exempt.