docs: private Combine resolveCombine(FunctionRef fun) {
          if (fun.getOperand().isEmpty() || fun.getOperand().size() > 2) {
              throw new IllegalArgumentException("Could not resolve call to system operator Combine.  Expected 1 or 2 arguments.");
          }
          final Combine combine = of.createCombine().withSource(fun.getOperand().get(0));
          if (fun.getOperand().size() == 2) {
              combine.setSeparator(fun.getOperand().get(1));
          }
          builder.resolveCall("System", "Combine", new CombineInvocation(combine));
          return combine;
      }