pg:start_link(), spawn(fun() -> pg:monitor(a), pg:join(a,self()) end).

=ERROR REPORT==== 5-Sep-2023::09:05:49.097826 ===
** Generic server pg terminating
** Last message in was {'DOWN',#Ref<0.1929657013.2338848804.134906>,process,
                               <0.107.0>,normal}
** When Server state == {state,pg,
                               #{<0.107.0> =>
                                     {#Ref<0.1929657013.2338848804.134908>,
                                      [a]}},
                               #{},#{},
                               #{#Ref<0.1929657013.2338848804.134906> =>
                                     {<0.107.0>,a}},
                               #{a =>
                                     [{<0.107.0>,
                                       #Ref<0.1929657013.2338848804.134906>}]}}
** Reason for termination ==
** {{case_clause,{{#Ref<0.1929657013.2338848804.134908>,[a]},#{}}},
    [{pg,handle_info,2,[{file,"pg.erl"},{line,407}]},
     {gen_server,try_handle_info,3,[{file,"gen_server.erl"},{line,1077}]},
     {gen_server,handle_msg,6,[{file,"gen_server.erl"},{line,1165}]},
     {proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,241}]}]}

<0.107.0>
=CRASH REPORT==== 5-Sep-2023::09:05:49.098435 ===
  crasher:
    initial call: pg:init/1
    pid: <0.106.0>
    registered_name: pg
    exception error: no case clause matching
                     {{#Ref<0.1929657013.2338848804.134908>,[a]},#{}}
      in function  pg:handle_info/2 (pg.erl, line 407)
      in call from gen_server:try_handle_info/3 (gen_server.erl, line 1077)
      in call from gen_server:handle_msg/6 (gen_server.erl, line 1165)
    ancestors: [<0.105.0>,<0.88.0>,<0.70.0>,<0.65.0>,<0.69.0>,<0.64.0>,
                  kernel_sup,<0.47.0>]
    message_queue_len: 1
    messages: [{'DOWN',#Ref<0.1929657013.2338848804.134908>,process,
                          <0.107.0>,normal}]
    links: [<0.105.0>]
    dictionary: []
    trap_exit: false
    status: running
    heap_size: 376
    stack_size: 28
    reductions: 7576
  neighbours:
    neighbour:
      pid: <0.105.0>
      registered_name: []
      initial_call: {erlang,apply,2}
      current_function: {io,execute_request,3}
      ancestors: [<0.88.0>,<0.70.0>,<0.65.0>,<0.69.0>,<0.64.0>,kernel_sup,
                  <0.47.0>]
      message_queue_len: 0
      links: [<0.88.0>,<0.106.0>]
      trap_exit: false
      status: waiting
      heap_size: 987
      stack_size: 27
      reductions: 4407
      current_stacktrace: [{io,execute_request,3,[{file,"io.erl"},{line,607}]},
                  {shell,exprs,7,[{file,"shell.erl"},{line,787}]},
                  {shell,eval_exprs,7,[{file,"shell.erl"},{line,736}]},
                  {shell,eval_loop,4,[{file,"shell.erl"},{line,721}]}]
** exception error: no case clause matching {{#Ref<0.1929657013.2338848804.134908>,[a]},#{}}
     in function  pg:handle_info/2 (pg.erl, line 407)
     in call from gen_server:try_handle_info/3 (gen_server.erl, line 1077)
     in call from gen_server:handle_msg/6 (gen_server.erl, line 1165)
     in call from proc_lib:init_p_do_apply/3 (proc_lib.erl, line 241)