xor - logical xor operator.
bool1 bool2 xor -> bool
true false xor -> true
true true xor -> false
false false xor -> false
For booleans, xor returns true, if either,
but not both of the arguments are true and false
otherwise.
/builddir/build/BUILD/nest-simulator-3.3/nest-simulator-3.3/sli/slimath.cc