1.6.0 and guardians

[Dirk Herrmann]

Hello together,

since we are approaching 1.6.0, I suddenly remember a point that might be
important for 1.6.0:  Michael has done a great re-implementation of
guardians as it became clear that Dybvig's definition of guardians has
some weak points.  In that re-implementation round, however, Michael has
introduced two kinds of guardians for experimental reasons.

IMO, we should revise the interface or at least add a note about the
current interface to be experimental.

Best regards
Dirk Herrmann