docs: public ValueType popValue() throws DataflowAnalysisException {
          if (!isValid()) {
              throw new DataflowAnalysisException("accessing top or bottom frame");
          }
          if (slotList.size() == numLocals) {
              throw new DataflowAnalysisException("operand stack empty");
          }
          return slotList.remove(slotList.size() - 1);
      }