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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def file(ui, repo, clname, pat, *pats, **opts):
 	"""assign files to or remove files from a change list 	Assign files to or (with -d) remove files from a change list. 	The -d option only removes files from the change list. 	It does not edit them or remove them from the repository. 	"""

	if codereview_disabled:
		raise hg_util.Abort(codereview_disabled)

	pats = tuple([pat] + list(pats))
	if not GoodCLName(clname):
		return "invalid CL name " + clname

	dirty = {}
	cl, err = LoadCL(ui, repo, clname, web=False)
	if err != '':
		return err
	if not cl.local:
		return "cannot change non-local CL " + clname

	files = ChangedFiles(ui, repo, pats)

	if opts["delete"]:
		oldfiles = Intersect(files, cl.files)
		if oldfiles:
			if not ui.quiet:
				ui.status("# Removing files from CL.  To undo:\n")
				ui.status("#	cd %s\n" % (repo.root))
				for f in oldfiles:
					ui.status("#	hg file %s %s\n" % (cl.name, f))
			cl.files = Sub(cl.files, oldfiles)
			cl.Flush(ui, repo)
		else:
			ui.status("no such files in CL")
		return

	if not files:
		return "no such modified files"

	files = Sub(files, cl.files)
	taken = Taken(ui, repo)
	warned = False
	for f in files:
		if f in taken:
			if not warned and not ui.quiet:
				ui.status("# Taking files from other CLs.  To undo:\n")
				ui.status("#	cd %s\n" % (repo.root))
				warned = True
			ocl = taken[f]
			if not ui.quiet:
				ui.status("#	hg file %s %s\n" % (ocl.name, f))
			if ocl not in dirty:
				ocl.files = Sub(ocl.files, files)
				dirty[ocl] = True
	cl.files = Add(cl.files, files)
	dirty[cl] = True
	for d, _ in dirty.items():
		d.Flush(ui, repo)
	return