+This is again identical as far as mimplid/isamux is concerned, with, again,
+a different kind of decision-making on traps. It was pointed out that
+if the mimplid/isamux is left unaltered when a trap occurs, switching over
+from one ISA to another inside a trap and dropping down to a different
+ISA in U-Mode is made slightly challenging by virtue of the fact that, when
+the trap changes the ISA, from that point onwards it *has to run that ISA
+inside the trap*. This may involve extra code-paths (branches) to require
+multiple different exit points from the trap.
+
+Whilst again it is quite unlikely that this scenario will arise (due to
+it being unlikely that the Base ISA will change significantly between
+(stable, approved) revisions to the RV Standard, the possibility cannot
+entirely be ruled out.
+
+So this idea is a hybrid of the above two: there is a default ISA for
+M-Mode and S-Mode, however in each it is possible to *set* that default.
+
+The idea has not yet been fully analysed and needs further discussion.
+