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**Background**: [ContainsAttestation](https://github.com/prysmaticlabs/prysm/blob/master/beacon-chain/attestation/service.go#L69) checks if a single attester exists in an attestation. Currently, the function accepts a bitfield that's assumed to be zero-filled except for the index which represents the single attester.
**Requirements**: Modify the function to accept the attester's validator index rather than a bitfield. Ensure that a bitfield isn't being created unnecessarily to represent a single validator.