sentences:  Narvasa served as associate justice until his appointment as chief justice on December 1, 1991, also by President Aquino.Narvasa administered the oath of office during the inaugurations of Philippine presidents Fidel V. Ramos and Joseph Ejercito Estrada.After his retirement in 1998, he was appointed by then President Joseph Ejercito Estrada to serve as chairman of the Preparatory Commission for Constitutional Reform, an independent commission formed to study the basic charter and suggest potentially positive modifications.