class_docstr: A map that creates new keys for missing key access.
  
  Considers keys of the form "<ident> <name>" to produce
  new symbols "<name>_<index>", where "index" is an incrementing integer
  corresponding to <name>.
  
  Inlines the approach taken by :class:`sqlalchemy.util.PopulateDict` which
  is otherwise usually used for this type of operation.
code: def __missing__(self, key: str) -> str:
      (ident, derived) = key.split(" ", 1)
      anonymous_counter = self.get(derived, 1)
      self[derived] = anonymous_counter + 1  # type: ignore
      value = f"{derived}_{anonymous_counter}"
      self[key] = value
      return value
  
file_path: sqlalchemy/lib/sqlalchemy/sql/_py_util.py
imports: from __future__ import annotations
  import typing
  from typing import Any
  from typing import Dict
  from typing import Tuple
  from typing import Union
  from ..util.typing import Literal
label_desc: Use image node_id 1 for calling the prefix_anon_map obj's underlying member method code with example usage: obj.__missing__(key) and returns: value