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