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.

@precond
@oparam{sourceLength}, @oparam{startPos}, and @oparam{numberToExtract}
are not negative.
@end precond

And here is a post condition example:

@postcond
After termination, evaluation of the procedure has done all the right
things and none of the wrong things.  In other words, the procedure is
@emph{perfect}, and to hell with anyone who questions this fact.
@end postcond
