In the DEC Processor Reference Manual, this is documented as "result is
indeterminate", and there's a note in the ITS KL microcode implying that
the default KL behaviour was different from the KA.
Muddle 54 uses this to drop a temporary stack frame from TP in a few
places. If it doesn't work properly, one obvious result is that READing
bracketed expressions leaves garbage on the TP stack -- e.g. typing
<QUIT> at the interpreter will print a FIX instead.
Muddle 56 replaces this with MOVE TP,(TP), so do that as a new patch.
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