Multiple-implementation libraries
=================================

It is already possible to depend on a libraryspec.

    \depends string 0.1   # <--- lets assume that string is a libraryspec

But there are some problems here:

- How to specify a (name,version) of the implementation?
- Where should this be specified?
    - in the main module?  (if so, how to handle conflicts between libraries?)
    - in the main module of the executable?
    - via a symlink?
    - dynamically selected via some environment varariable?
        e.g. "LD_SLUL_libxxx=yyy"
- How to specify that an \api_def in the impl is for that libraryspec?
    \api_def_for string 0.1 1111111111111111111111