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thanks to Christian Doczkal <doczkal@ps.uni-saarland.de> for the Makefile snippet!

We can use a named representation because we only substitute closed values. This
idea is borrowed from Pierce's Software Foundations.

The named representation has the following advantages:
* Programs are much better readable than those using De Bruijn indexes.
* Substitutions on closed terms (where all variables are explicit strings) can
  be performed by a mere simpl instead of Autosubst's asimpl. The performance
  of simpl seems better than asimpl.
* Syntactic sugar refolds better.

We should not depend on unreleased, git-only versions.
This reverts commit ee948028.