inserts clear() and swap().
inserts clear() and swap().

hiroyuki-seki/hryky-codebase,hiroyuki-seki/hryky-codebase,hiroyuki-seki/hryky-codebase,hiroyuki-seki/hryky-codebase,hiroyuki-seki/hryky-codebase,hiroyuki-seki/hryky-codebase