INDUCTIVE DEFINITION or3 () [ :Prop, :Prop, :Prop ]
OF ARITY Prop
BUILT FROM:
     or3_intro0: P0→(or3 P0 P1 P2)
   | or3_intro1: P1→(or3 P0 P1 P2)
   | or3_intro2: P2→(or3 P0 P1 P2)