ZMlog is a ZMstandardLog, instantiated in ZMexceptions.cc, using stderr as
its destination.

ZMxTop is declared in ZMexceptions.cc, instatiated with ZMlog as its log.

To change the destination of the default logging, a framework writer
would instantiate another ZMstandardLog with, say, a file name.  Then
ZMxTop.setLogger(thatLog) is called.