code: def to_single_stage_lockfile(stage):
      """Validate schema on each serialization."""
      e = _to_single_stage_lockfile(stage)
      assert Schema(e)
      return e
  
imports: from collections import OrderedDict
  import pytest
  from voluptuous import Schema
  from dvc.dvcfile import PROJECT_FILE
  from dvc.schema import LOCK_FILE_STAGE_SCHEMA, LOCKFILE_STAGES_SCHEMA
  from dvc.stage import PipelineStage, create_stage
  from dvc.stage.serialize import DEFAULT_PARAMS_FILE, to_lockfile
  from dvc.stage.serialize import to_single_stage_lockfile
  from dvc.stage.utils import split_params_deps
  from dvc_data.hashfile.hash_info import HashInfo
label_desc: Use image node_id 1 for calling a global function with example usage: to_single_stage_lockfile(stage) and returns: e