addStruct = (store, struct) => {
  let structs = store.clients.get(struct.id.client);

  if (structs === undefined) {
    structs = [];
    store.clients.set(struct.id.client, structs);
  } else {
    const lastStruct = structs[structs.length - 1];

    if (lastStruct.id.clock + lastStruct.length !== struct.id.clock) {
      throw error.unexpectedCase();
    }
  }

  structs.push(struct);
}
@param {StructStore} store
@param {GC|Item} struct

@private
@function