combineN method

@override T combineN(T a, int n)

Only apply combine operation the first time:

• n == 1, then return a
• Otherwise return combine(a, a)

Implementation

@override
T combineN(T a, int n) {
  if (n <= 0) {
    throw const FormatException(
        "Repeated combining for bands must have n > 0");
  }

  return n == 1 ? a : combine(a, a);
}