Specify identifier variables for multiple-membership and hierarchical structures
Source:R/helpers.R
id.RdHelper function used within mm and hm to specify
the identifier variables that define memberships and nesting structures. In
multiple-membership models, id() links member-level units (e.g., party
IDs) to group-level units (e.g., government IDs). In hierarchical models,
id() specifies the nesting-level identifier (e.g., country ID).