docs: public Collection<String> calculateUDFDependees() {
          List<String> answer = new ArrayList<>();
          Collection<AbstractExpression> fCalls = findAllSubexpressionsOfClass(FunctionExpression.class);
          for (AbstractExpression fCall : fCalls) {
              FunctionExpression fexpr = (FunctionExpression)fCall;
              if (fexpr.isUserDefined()) {
                  answer.add(fexpr.getFunctionName());
              }
          }
          return answer;
      }