SIMPNCEXPT is not careful about simplifying ^^
(MNCEXPT, noncommutative exponent).
aa ^^ bb simplifies to aa ^ bb when aa is a scalar or
constant, (e.g. %e ^^ matrix => %e ^ matrix) but that's
not generally correct.
We can debate whether SIMPNCEXPT ought to go to the
expense of executing the matrix exponential code, but
if not that, then SIMPNCEXPT should at least return a