code: public T visitOperandDef(OperandDef elm, C context) {
          if (elm.getOperandTypeSpecifier() != null) {
              visitElement(elm.getOperandTypeSpecifier(), context);
          }
          return null;
      }
docs: Visit a OperandDef. This method will be called for
  every node in the tree that is a OperandDef.
  
  @param elm     the ELM tree
  @param context the context passed to the visitor
  @return the visitor result