In discussion, Pacino and co. are studying the "*G* of Edward's heirs the murderer shall be," and decide, since it's supposed to refer to Clarence, that they'll change it to "'C" of Edward's heir's." The problem is, since characters in the play are referred to both by their name and by their title, the prophecy very deliberately refers to Richard, Duke of GLOUCESTER and GEORGE, Duke of Clarence. With "G" the prophecy is true. If you change it to "C" the prophecy becomes false, and can no longer refer to two people.