INDUCTIVE DEFINITION pc3_left () [ :C ]
OF ARITY TTProp
BUILT FROM:
     pc3_left_r: t:T.(pc3_left c t t)
   | pc3_left_ur: t1:T.t2:T.(pr2 c t1 t2)t3:T.(pc3_left c t2 t3)(pc3_left c t1 t3)
   | pc3_left_ux: t1:T.t2:T.(pr2 c t1 t2)t3:T.(pc3_left c t1 t3)(pc3_left c t2 t3)