Function check_only_for_signature 

pub fn check_only_for_signature(
    cmd: Box<SignedCommand>,
) -> Result<UserCommand, Box<SignedCommand>>

https://github.com/MinaProtocol/mina/blob/05c2f73d0f6e4f1341286843814ce02dcb3919e0/src/lib/mina_base/signed_command.ml#L436