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