 Complete the following phrase:  Encyclicals indicate high papal priority for an issue at a given time. Pontiffs define when, and under which circumstances, encyclicals should be issued. They may choose to issue an apostolic constitution, bull, encyclical, apostolic letter or give a papal speech. Popes have differed on the use of encyclicals: on the issue of birth control and contraception, Pope Pius XI issued the encyclical Casti connubii, while Pope Pius XII gave a speech to midwives and the medical profession, clarifying the position of the church on the issue. Pope Paul VI published an encyclical Humanae vitae on the same topic. On matters of war and peace, Pope Pius XII issued ten encyclicals, mostly after 1945, three of them protesting the Soviet invasion of Hungary in order to crack down on the 