removeAt method
Implementation
RSeq<A> removeAt(int idx) {
if (0 <= idx && idx < length) {
if (idx == 0) {
return tail();
} else {
final (a, b) = splitAt(idx);
return a.concat(b.tail());
}
} else {
throw RangeError('$idx is out of bounds (min 0, max ${length - 1})');
}
}