docs: def _format_arg(arg):
      """Validate one exception specification contract tuple/list."""
      # Check that the argument conforms to one of the acceptable types, a string
      # (when the default exception type is used), an exception type (when the
      # default exception message is used) or a list/tuple to specify both
      # exception type and exception message
      if not (_isexception(arg) or isinstance(arg, (str, tuple, list))):
          raise TypeError("Illegal custom contract exception definition")
      # Check that when the argument is a list or tuple, they only have at most
      # 2 items, the exception type and the exception message
      if isinstance(arg, (tuple, list)) and ((not arg) or (len(arg) > 2)):
          raise TypeError("Illegal custom contract exception definition")
      # When only an exception message is given (and the default RuntimeError
      # exception type is used), check that the message is not empty
      if isinstance(arg, str) and (not arg):
          raise ValueError("Empty custom contract exception message")
      # When only an exception message is defined,
      # use the default exception type
      if isinstance(arg, str):
          return {"msg": arg, "type": RuntimeError}
      # When only an exception type is defined,
      # use the default exception message
      if _isexception(arg):
          return {"msg": "Argument `*[argument_name]*` is not valid", "type": arg}
      # If a list/tuple definition is used, check that if is not a string, it is
      # a valid exception type (i.e. that it actually raises an exception)
      if (len(arg) == 1) and (not isinstance(arg[0], str)) and (not _isexception(arg[0])):
          raise TypeError("Illegal custom contract exception definition")
      if (len(arg) == 2) and (
          not (
              (isinstance(arg[0], str) and _isexception(arg[1]))
              or (isinstance(arg[1], str) and _isexception(arg[0]))
          )
      ):
          raise TypeError("Illegal custom contract exception definition")
      # Check that the exception definition has a non-empty exception message
      # when a list/tuple definition is used
      if (len(arg) == 1) and isinstance(arg[0], str) and (not arg[0]):
          raise ValueError("Empty custom contract exception message")
      if (len(arg) == 2) and (
          (isinstance(arg[0], str) and (not arg[0]))
          or (isinstance(arg[1], str) and (not arg[1]))
      ):
          raise ValueError("Empty custom contract exception message")
      # Return conforming dictionary with default exception type and exception
      # message applied (if necessary)
      if len(arg) == 1:
          return {
              "msg": (
                  arg[0]
                  if isinstance(arg[0], str)
                  else "Argument `*[argument_name]*` is not valid"
              ),
              "type": arg[0] if _isexception(arg[0]) else RuntimeError,
          }
      # len(arg) == 2
      return {
          "msg": arg[0] if isinstance(arg[0], str) else arg[1],
          "type": arg[0] if _isexception(arg[0]) else arg[1],
      }