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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 from_string(cls, public_key):
 """Construct an Verifier instance from a public key or public certificate string. Args: public_key (Union[str, bytes]):
 The public key in PEM format or the x509 public key certificate. Returns: Verifier: The constructed verifier. Raises: ValueError: If the public key can't be parsed. """

        public_key_data = _helpers.to_bytes(public_key)

        if _CERTIFICATE_MARKER in public_key_data:
            cert = cryptography.x509.load_pem_x509_certificate(
                public_key_data, _BACKEND)
            pubkey = cert.public_key()

        else:
            pubkey = serialization.load_pem_public_key(
                public_key_data, _BACKEND)

        return cls(pubkey)