The block commands @precond and @postcond are not part of Texinfo.
They are an extension used in oberon-doc to describe the pre and post
conditions of a procedure.

*Pre-condition*: SOURCELENGTH, STARTPOS, and NUMBERTOEXTRACT are not
negative.

   And here is a post condition example:

*Pre-condition*: After termination, evaluation of the procedure has
done all the right things and none of the wrong things.  In other
words, the procedure is _perfect_, and to hell with anyone who
questions this fact.

