D.17.4 Additional umf notes

The C language API to umf provides very many less frequently used features that are not part of the virtual code interface, some of which could be added by minor modifications to the file umf.c in the avram source tree.

A set of dl_* and zl_* functions orthogonal to those presently accessible would enable matrices having billions of rows or columns by using long integers, but memory requirements on the virtual code side for problems of that scale are probably prohibitive for the foreseeable future.