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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 export_certificate(ctx, slot, format, certificate):
 """ Export a X.509 certificate. Reads a certificate from one of the slots on the YubiKey. \b SLOT PIV slot to read certificate from. CERTIFICATE File to write certificate to. Use '-' to use stdout. """

    controller = ctx.obj['controller']
    try:
        cert = controller.read_certificate(slot)
    except APDUError as e:
        if e.sw == SW.NOT_FOUND:
            ctx.fail('No certificate found.')
        else:
            logger.error('Failed to read certificate from slot %s', slot,
                         exc_info=e)
    certificate.write(cert.public_bytes(encoding=format))