the proof TST:

(DECL ((P CONSTANT TRUTHVAL UNIVERSAL) ) NIL)
1. context: 
P has syntype CONSTANT, type TRUTHVAL and has sort UNIVERSAL.

(ASSUME P (1. ))
2. P
context: (1.)
dependencies: (2.)

(⊃I 2. 2.)
3. P⊃P
context: (1.)
dependencies: NIL