The index_map argument can take several forms.
This function figures out (for a given a_mat)
the index maps for rows (first data frame in the return list)
and columns (second data frame in the return list).
Value
A list of two data frames.
The first data frame is the index map for the rows of a_mat.
The second data frame is the index map for the columns of a_mat.
Details
ind_maps can be a single data frame,
in which case the single data frame will be applied
to both rows and columns of a_mat.
ind_maps can also be a 2-item list,
in which case the first item is applied to rows and the
second item is applied to columns.
Finally, ind_maps can be a named list of length 2 or more,
in which case the names are interpreted as row or column types.
Names in the ind_maps list are matched to row and column types
and applied as required.
This is a non-exported function meant only for internal use.