PREUN
/bin/sh
if [ $1 -eq 0 ]; then /usr/sbin/update-alternatives --remove yacc /usr/bin/byacc fi
POSTIN
/bin/sh
/usr/sbin/update-alternatives --install /usr/bin/yacc yacc /usr/bin/byacc 20
Triggers
byacc <= 1.9-16mdk
/bin/sh
/usr/sbin/update-alternatives --install /usr/bin/yacc yacc /usr/bin/byacc 20