public static Call<Void> newVoidCall(List<Call<Void>> calls) {
    if (calls.isEmpty()) throw new IllegalArgumentException("calls were empty");
    if (calls.size() == 1) return calls.get(0);
    return new AggregateVoidCall(calls);
  }
A call that blocks on others to complete before invoking a callback or returning from {@link
#execute()}. The first error will be returned upstream, later ones will be suppressed.

@param <I> the type of returned from {@link Call#execute()}
@param <O> the type representing the aggregate success value
zipkin/src/main/java/zipkin2/internal/AggregateCall.java