Should additional list columns be dropped? By default,
unnest will drop them if unnesting the specified columns requires
the rows to be duplicated.
.id
Data frame idenfier - if supplied, will create a new column
with name .id, giving a unique identifer. This is most useful if
the list column is named.
.sep
If non-NULL, the names of unnested data frame columns
will combine the name of the original list-col with the names from
nested data frame, separated by .sep.