Fix append operator. #11582

Job	Run time
build_details	3s
build_js	2m 8s
build_opam (4.14.x)	9m 4s
build_opam (5.2.x)	5m 10s
build_no_depopts	6m 5s
build_win32 (x64)	18m 8s
build_posix (debian_trixie, amd64)	13m 3s
lezer_parse	53s
build_posix (debian_bookworm, amd64)	14m 26s
run_tests (@citest)	12m 38s
build_posix (ubuntu_oracular, amd64)	15m 28s
tree_sitter_parse	50s
run_tests (@doctest)	7m 31s
build_posix (ubuntu_noble, amd64)	14m 48s
update_doc	0s
run_tests (@mediatest)	11m 47s
build_posix (alpine, amd64)	11m 32s
fetch_s3_artifacts	20s
build_docker_alpine_minimal (amd64)	53s
build_docker_minimal (amd64)	2m 11s
build_docker_alpine (amd64)	54s
build_docker (amd64)	2m 4s
build_docker_release	0s
update_release	0s
build_docker_release-minimal	0s
	2h 29m 56s

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