These functions return information about the "current" group or "current"
variable, so only work inside specific contexts like summarise() and
mutate()
Context-dependent: n() returns an integer; cur_group_id() returns
an integer; cur_group() returns a tibble; cur_data() and cur_data_all()
return tibbles; cur_column() returns a character string.
not used; present for compatibility with the generic
n() gives the current group size.
cur_data() gives the current data for the current group (excluding
grouping variables).
cur_data_all() gives the current data for the current group (including
grouping variables)
cur_group() gives the group keys, a tibble with one row and one column
for each grouping variable.
cur_group_id() gives a unique numeric identifier for the current group.
cur_column() gives the name of the current column (in dplyr::across() only).
See dplyr::group_data() for equivalent functions that return values for all
groups.