code: @Override
      public void sawOpcode(int seen) {
          if (seen == Const.PUTFIELD) {
              QMethod methodInfo = new QMethod(getMethodName(), getMethodSig());
              Map<Map.Entry<String, String>, SourceLineAnnotation> fields = methodWrites.get(methodInfo);
              if (fields == null) {
                  fields = new HashMap<>();
                  methodWrites.put(methodInfo, fields);
              }
              String fieldName = getNameConstantOperand();
              String fieldSig = getSigConstantOperand();
  
              FieldAnnotation fa = new FieldAnnotation(getDottedClassName(), fieldName, fieldSig, false);
              fieldAnnotations.put(fieldName, fa);
              fields.put(new AbstractMap.SimpleImmutableEntry(fieldName, fieldSig), SourceLineAnnotation.fromVisitedInstruction(this));
          }
      }
docs: implements the visitor to record storing of fields, and where they occur
  
  @param seen
  the currently parsed opcode