Function membership_head_stream_prefix 

pub fn membership_head_stream_prefix(
    author: &str,
    author_owner_grant: &MembershipGrantId,
    stream_id: AuthorStreamId,
) -> String

Everything one author stream’s head slots share, up to the sequence number.

A head’s slot is named by its coordinate alone, so a provider can enumerate a whole stream under this prefix instead of a reader learning each slot from the head before it. The founder’s first head is the exception: it is written under founder_membership_head_semantic_prefix before the founder has a grant to name, so a listing here starts the founder’s stream at sequence 2.