sudo pkg_add expat
Error from ftp://ftp.openbsd.org/pub/OpenBSD/4.0/packages/i386/:
Unknown command.
Collision: the following files already exist
        /usr/local/bin/xmlwf (same md5)
        /usr/local/include/expat.h (same md5)
        /usr/local/include/expat_external.h (same md5)
        /usr/local/lib/libexpat.a (same md5)
        /usr/local/lib/libexpat.la (same md5)
        /usr/local/lib/libexpat.so.6.0 (different md5)
        /usr/local/man/man1/xmlwf.1 (same md5)
        /usr/local/share/doc/expat/reference.html (same md5)
        /usr/local/share/doc/expat/style.css (same md5)
        /usr/local/share/doc/expat/valid-xhtml10.png (same md5)
        /usr/local/share/examples/expat/elements.c (same md5)
        /usr/local/share/examples/expat/outline.c (same md5)
/usr/sbin/pkg_add: fatal issues in installing expat-2.0.0