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/usr/share/X11/locale/ja/Compose is in libx11-data 2:1.6.3-1ubuntu2.1.

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#
# ja Compose Sequence
#
# Sequence Definition
#
#
# This file currently has no entries.  It appears that a compose file (even
# just an empty one) is required for the appropriate keysyms to work for
# this encoding.
#
# <Multi_key> Means <Compose>
# Special Character

# End of Sequence Definition