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This abstract contract provides a fallback function that delegates all calls to another contract using the EVM instruction delegatecall. We refer to the second contract as the implementation behind the proxy, and it has to be specified by overriding the virtual {_implementation} function. Additionally, delegation to the implementation can be triggered manually through the {_fallback} function, or to a different contract through the {_delegate} function. The success and return data of the delegated call will be returned back to the caller of the proxy.