docs: def ok(prompt='OK ', loc={}, glo={}, cmd=""):
      '''
      Invoke the peforth interpreter.
      An statement: peforth.ok(prompt='OK ', loc=locals(), glo=globals(), cmd="") 
      is like a breakpoint. The prompt indicates which breakpoint it is if there are 
      many. Arguments loc (locals) and glo (globals) along with the prompt are the 
      debuggee's informations that is packed as a tuple (loc,glo,prompt) left on TOS 
      of the FORTH vm when the breakpoint is called. Replace the loc=locals() with
      loc=dict(locals()) to get a snapshot copy instead of a reference, as well as 
      the glo. 'exit' command to stop debugging.
      '''
      if loc or glo: vm.push((loc,glo,prompt))  # parent's data
      while True:
          if cmd == "":                                    #
              if vm.tick('accept') and not vm.multiple:    # Input can be single line (default) or
                  vm.execute('accept')                     # multiple lines. Press Ctrl-D to toggle
                  cmd = vm.pop().strip()                   # between the two modes. Place a Ctrl-D
              elif vm.tick('<accept>') and vm.multiple:    # before the last <Enter> key to end the
                  vm.execute('<accept>')                   # input when in multiple-line mode.
                  cmd = vm.pop().strip()                   #
              else:                                        #
                  cmd = input("").strip()                  #
  
          # pass the command line to forth VM
          if cmd == "":
              print(prompt, end="")
              continue
          elif cmd == chr(4):
              vm.multiple = not vm.multiple
              if not vm.multiple: print(prompt, end="")
          else:
              vm.dictate(cmd)
              if vm.multiple: vm.multiple = False # switch back to the normal mode
              print(prompt, end="")
          cmd = ""
          # Master switch vm.exit is a flag of boolean. When it's True
          # then exit to the caller that usually is python interpreter.
          if vm.exit:
              vm.exit = False # Avoid exit immediately when called again
              break
      return(vm) # support function cascade