Countable instance for vec, and rename `vec_to_list_of_list` into `vec_to_list_to_vec`
This is an alternative to !114 (closed), which does not involve defining a partial function from lists to vectors, but makes use of the already existing function list_to_vec
.
Also I renamed vec_to_list_of_list
into vec_to_list_to_vec
. The former appears to be a leftover of the big X_of_Y
into Y_to_X
rename.