code: def merge(
      config: Config,
      revisions: _RevIdType,
      message: Optional[str] = None,
      branch_label: Optional[_RevIdType] = None,
      rev_id: Optional[str] = None,
  ) -> Optional[Script]:
      """Merge two revisions together.  Creates a new migration file.
  
      :param config: a :class:`.Config` instance
  
      :param message: string message to apply to the revision
  
      :param branch_label: string label name to apply to the new revision
  
      :param rev_id: hardcoded revision identifier instead of generating a new
       one.
  
      .. seealso::
  
          :ref:`branches`
  
      """
  
      script = ScriptDirectory.from_config(config)
      template_args = {
          "config": config  # Let templates use config for
          # e.g. multiple databases
      }
  
      environment = util.asbool(
          config.get_main_option("revision_environment")
      )
  
      if environment:
  
          def nothing(rev, context):
              return []
  
          with EnvironmentContext(
              config,
              script,
              fn=nothing,
              as_sql=False,
              template_args=template_args,
          ):
              script.run_env()
  
      return script.generate_revision(
          rev_id or util.rev_id(),
          message,
          refresh=True,
          head=revisions,
          branch_labels=branch_label,
          **template_args,  # type:ignore[arg-type]
      )
  
imports: from __future__ import annotations
  import os
  from typing import List
  from typing import Optional
  from typing import TYPE_CHECKING
  from typing import Union
  from .None import autogenerate
  from .None import util
  from .runtime.environment import EnvironmentContext
  from .script import ScriptDirectory
label_desc: Use image node_id 5 for calling a global function with example usage: merge(config, revisions, message, branch_label, rev_id) and returns: script, list