“ The original plaintiff in Carden was the limited partnership; diversity jurisdiction, then, depended upon whether complete diversity of citizenship existed at the time the action was commenced. But nothing in Carden suggests any change in the well-established rule that diversity of citizenship is assessed at the time the action is filed. We have consistently held that if jurisdiction exists at the time an action is commenced, such jurisdiction may not be divested by subsequent events. ”

“ Our decision last Term in Carden considered whether the citizenship of limited partners must be taken into account in determining whether diversity jurisdiction exists in an action brought by a limited partnership. ”

“ Petitioners claimed that respondent had failed to pay the price for natural gas agreed upon in their contract, and sought both declaratory relief to establish the contract price and damages for past underpayments. Petitioners based federal jurisdiction upon diversity of citizenship. ”