What is Declaration of Lex Talionis?

Early in the First English Civil War the Long Parliament threatened to retaliate in kind if the Royalists tried and executed John Lilburne and two other Parliamentary offices for treason. Lilburne later described this as the declaration of Lex Talionis, and it brought about a practical—rather than moral—mutual restraint by the parties to the war on how they treated prisoners of war.