Find which variables are unique at a given level in hierarchical data
get_unique_variables_by_level(data, ID_label, superset = NULL)
a data.frame
the ID label to split upon
Superset contains a vector of character strings that contain variables the modify level call is going to write. Some of these may be columns in the data frame, others might not be. If superset is specified, then we definitely only want to check those variables
a character vector enumerating the unique variables