Function field_string_lists 

fn field_string_lists(
    table: &LuaTable,
    field: &str,
    at: &str,
) -> Result<HashMap<String, Vec<String>>, String>

A header or query-parameter map. The canonical shape is a list of strings per key (that is what the script is handed), but headers["x-user"] = "alice" is the natural thing to write, so a bare scalar is accepted as a one-element list. Anything else names the offending key.