Copyright for r-cran-snowballc source package in Bionic

Format: https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/
Upstream-Name: SnowballC
Upstream-Contact: Milan Bouchet-Valat 
Source: https://cran.r-project.org/package=SnowballC

Files: *
Copyright: 2013-2014 Milan Bouchet-Valat
License: BSD-2-clause

Files: src/*.c src/*.h
Copyright: 2001 Dr Martin Porter
License: BSD-2-clause

Files: debian/*
Copyright: 2017 Andreas Tille 
License: BSD-2-clause

License: BSD-2-clause
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions are
 met:
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    Redistributions of source code must retain the above copyright
    notice, this list of conditions and the following disclaimer.
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    Redistributions in binary form must reproduce the above copyright
    notice, this list of conditions and the following disclaimer in
    the documentation and/or other materials provided with the
    distribution.
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 A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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