(command.CommandList
  children: [
    (C {<set>} {<-ex>})
    (C {<source>} {($ Id.VSub_DollarName TRAVIS_BUILD_DIR) <'/ci/travis_env_common.sh'>})
    (C {<pushd>} {($ Id.VSub_DollarName ARROW_JS_DIR)})
    (C {<npm>} {<install>})
    (C {<popd>})
  ]
)