Function prepare 

pub fn prepare(
    store: &impl ObjectSource,
    scope: CollaborationScope,
    state: &State,
    collaboration_frontier: ContentHash,
    roots: Vec<ReferenceClosure>,
    seeds: Vec<Seed>,
    symbol: impl FnMut(&FileResolution, &FileResolution, &Blob, &Blob, &str) -> Result<(String, ResolutionStatus)>,
) -> Result<Prepared>