combineN method
Return a appended to itself n times. If n is negative, then
this returns inverse(a) appended to itself n times.
Implementation
@override
T combineN(T a, int n) {
if (n > 0) {
return _repeatedCombineN(a, n);
} else if (n == 0) {
return empty;
}
return _repeatedCombineN(inverse(a), -n);
}