let create     ?(max_processors = 1)     ?gatk_jar_location     ?mutect_jar_location     ?netmhc_tool_locations     ?pyensembl_cache_dir     ?run_program ?toolkit ?b37 uri =   let open KEDSL in   let host = Host.parse (uri // "ketrew_playground") in   let meta_playground = Uri.of_string uri |> Uri.path in   let run_program =     match run_program with     | None -> default_run_program ~host     | Some r -> r   in   let toolkit =     Option.value toolkit       ~default:(Tool_providers.default_toolkit ()                   ~run_program                   ~host ~install_tools_path:(meta_playground // "install-tools")                   ?gatk_jar_location ?mutect_jar_location                   ?netmhc_tool_locations)   in   Machine.create (sprintf "ssh-box-%s" uri)     ~max_processors     ?pyensembl_cache_dir     ~get_reference_genome:(fun name ->         match name, b37 with         | name, Some some37 when name = Reference_genome.name some37 -> some37         | name, _ ->                   Download_reference_genomes.get_reference_genome name             ~toolkit ~host ~run_program             ~destination_path:(meta_playground // "reference-genome"))     ~host     ~toolkit     ~run_program     ~work_dir:(meta_playground // "work")