Manila, Sept. 16 -- A 25-year-old Senate precedent in which only 13 senators were needed to concur in four treaties surfaced Wednesday as a possible guide in resolving the disputed voting threshold in Vice President Sara Duterte's impeachment trial.

Senate Majority Leader Juan Miguel Zubiri presented excerpts from the Senate journal dated March 19, 2001 showing the same 13 senators voting in favor of four international agreements.

"Did the Senate violate the Constitution at this particular instance? Did we err in the approval of these four very important international treaties with just 13 members?" Zubiri asked the amici curiae (friends of the court).

The precedent drew attention because Article VII, Section 21 of the Constitution req...