Iof proof for Lemma decidable and:
1. P :
2. Q :
3. P
(
P)
4. Q
(Q)
(P 
Q) ((P Q))
by ((D 4)
CollapseTHEN (D 3))
C1:
C1: 3. P
C1: 4. Q
C1: (P Q) ((P Q))
C2:
C2: 3. P
C2: 4. Q
C2: (P Q) ((P Q))
C3:
C3: 3. P
C3: 4. Q
C3: (P Q) ((P Q))
C4:
C4: 3. P
C4: 4. Q
C4: (P Q) ((P Q))
C.