code: def expr_idxs(self, idxs: Optional[Iterable[Node]] = None):
      if idxs is None:
          idxs = [
              Variable(f"idx{i}", 0, s - 1)
              for i, s in enumerate(self.shape)
          ]
      idx = expr_idxs(self.views[-1], tuple(idxs))
      valid = expr_node_mask(
          self.views[-1], idxs_to_idx(self.views[-1].shape, tuple(idxs))
      )
      return self._expr_idx(idx, valid)
  
imports: from __future__ import annotations
  import functools, itertools, operator
  from dataclasses import dataclass
  from typing import Tuple, List, Optional, Dict, Set, cast, Union, Iterable
  from tinygrad.ops import MovementOps
  from tinygrad.helpers import prod, merge_dicts, getenv
  from tinygrad.shape.symbolic import Variable, MulNode, Node, SumNode, NumNode, sint
  from tinygrad.shape.view import View, _merge_dims
label_desc: Use image node_id 15 for calling the ShapeTracker obj's underlying member method code with example usage: obj.expr_idxs(idxs) and returns: self