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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _resolve_credential(self, credential):
 """Resolve credential from envars or credentials store. Args: credential (str):
 Credential to resolve. Returns: str or None: Resolved credential or ``None``. """

        if self._credentials_found_in_instance:
            return
        elif self._credentials_found_in_envars():
            return os.getenv('PAN_' + credential.upper())
        else:
            return self.storage.fetch_credential(
                credential=credential, profile=self.profile)