Function freeze 

pub fn freeze(
    project_state: &Path,
    unit: &str,
    f: &Freezable,
    approval_digest: &str,
    decided: Decided,
    producer: Producer,
) -> Result<PreFrozenAcceptance, String>

Freezes one requirement’s red set with the approval: one PreFrozenAcceptance (producer review) in the pre-frozen store, its sealed held-out set checked, and the commands and wording beside it.