/usr/include/claws-mail/common/ is in libclaws-mail-dev 3.14.1-3+b1.
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/usr/include/claws-mail/common/claws.h
/usr/include/claws-mail/common/defs.h
/usr/include/claws-mail/common/hooks.h
/usr/include/claws-mail/common/log.h
/usr/include/claws-mail/common/md5.h
/usr/include/claws-mail/common/mgutils.h
/usr/include/claws-mail/common/passcrypt.h
/usr/include/claws-mail/common/pkcs5_pbkdf2.h
/usr/include/claws-mail/common/plugin.h
/usr/include/claws-mail/common/prefs.h
/usr/include/claws-mail/common/progressindicator.h
/usr/include/claws-mail/common/quoted-printable.h
/usr/include/claws-mail/common/session.h
/usr/include/claws-mail/common/smtp.h
/usr/include/claws-mail/common/socket.h
/usr/include/claws-mail/common/ssl.h
/usr/include/claws-mail/common/ssl_certificate.h
/usr/include/claws-mail/common/string_match.h
/usr/include/claws-mail/common/stringtable.h
/usr/include/claws-mail/common/tags.h
/usr/include/claws-mail/common/template.h
/usr/include/claws-mail/common/timing.h
/usr/include/claws-mail/common/tlds.h
/usr/include/claws-mail/common/utils.h
/usr/include/claws-mail/common/uuencode.h
/usr/include/claws-mail/common/version.h
/usr/include/claws-mail/common/xml.h
/usr/include/claws-mail/common/xmlprops.h