let Al be QC-alphabet ; :: thesis: for l being Nat
for X being Subset of (CQC-WFF Al)
for f being FinSequence of st f is_a_proof_wrt X & 1 <= l & l <= len f holds
(f . l) `1 in Cn X

let l be Nat; :: thesis: for X being Subset of (CQC-WFF Al)
for f being FinSequence of st f is_a_proof_wrt X & 1 <= l & l <= len f holds
(f . l) `1 in Cn X

let X be Subset of (CQC-WFF Al); :: thesis: for f being FinSequence of st f is_a_proof_wrt X & 1 <= l & l <= len f holds
(f . l) `1 in Cn X

let f be FinSequence of ; :: thesis: ( f is_a_proof_wrt X & 1 <= l & l <= len f implies (f . l) `1 in Cn X )
assume that
A1: f is_a_proof_wrt X and
A2: ( 1 <= l & l <= len f ) ; :: thesis: (f . l) `1 in Cn X
for n being Nat st 1 <= n & n <= len f holds
(f . n) `1 in Cn X
proof
defpred S1[ Nat] means ( 1 <= \$1 & \$1 <= len f implies (f . \$1) `1 in Cn X );
A3: for n being Nat st ( for k being Nat st k < n holds
S1[k] ) holds
S1[n]
proof
let n be Nat; :: thesis: ( ( for k being Nat st k < n holds
S1[k] ) implies S1[n] )

assume A4: for k being Nat st k < n holds
S1[k] ; :: thesis: S1[n]
assume that
A5: 1 <= n and
A6: n <= len f ; :: thesis: (f . n) `1 in Cn X
A7: f,n is_a_correct_step_wrt X by A1, A5, A6;
now :: thesis: (f . n) `1 in Cn X
not not (f . n) `2 = 0 & ... & not (f . n) `2 = 9 by A5, A6, Th19;
per cases ) `2 = 0 or (f . n) `2 = 1 or (f . n) `2 = 2 or (f . n) `2 = 3 or (f . n) `2 = 4 or (f . n) `2 = 5 or (f . n) `2 = 6 or (f . n) `2 = 7 or (f . n) `2 = 8 or (f . n) `2 = 9 ) ;
suppose (f . n) `2 = 0 ; :: thesis: (f . n) `1 in Cn X
then A8: (f . n) `1 in X by ;
X c= Cn X by Th13;
hence (f . n) `1 in Cn X by A8; :: thesis: verum
end;
suppose (f . n) `2 = 1 ; :: thesis: (f . n) `1 in Cn X
then (f . n) `1 = VERUM Al by ;
hence (f . n) `1 in Cn X by Th2; :: thesis: verum
end;
suppose (f . n) `2 = 2 ; :: thesis: (f . n) `1 in Cn X
then ex p being Element of CQC-WFF Al st (f . n) `1 = (() => p) => p by ;
hence (f . n) `1 in Cn X by Th3; :: thesis: verum
end;
suppose (f . n) `2 = 3 ; :: thesis: (f . n) `1 in Cn X
then ex p, q being Element of CQC-WFF Al st (f . n) `1 = p => (() => q) by ;
hence (f . n) `1 in Cn X by Th4; :: thesis: verum
end;
suppose (f . n) `2 = 4 ; :: thesis: (f . n) `1 in Cn X
then ex p, q, r being Element of CQC-WFF Al st (f . n) `1 = (p => q) => (('not' (q '&' r)) => ('not' (p '&' r))) by ;
hence (f . n) `1 in Cn X by Th5; :: thesis: verum
end;
suppose (f . n) `2 = 5 ; :: thesis: (f . n) `1 in Cn X
then ex p, q being Element of CQC-WFF Al st (f . n) `1 = (p '&' q) => (q '&' p) by ;
hence (f . n) `1 in Cn X by Th6; :: thesis: verum
end;
suppose (f . n) `2 = 6 ; :: thesis: (f . n) `1 in Cn X
then ex p being Element of CQC-WFF Al ex x being bound_QC-variable of Al st (f . n) `1 = (All (x,p)) => p by ;
hence (f . n) `1 in Cn X by Th8; :: thesis: verum
end;
suppose (f . n) `2 = 7 ; :: thesis: (f . n) `1 in Cn X
then consider i, j being Nat, p, q being Element of CQC-WFF Al such that
A9: 1 <= i and
A10: i < n and
A11: 1 <= j and
A12: j < i and
A13: ( p = (f . j) `1 & q = (f . n) `1 & (f . i) `1 = p => q ) by ;
A14: j < n by ;
A15: i <= len f by ;
then j <= len f by ;
then A16: (f . j) `1 in Cn X by A4, A11, A14;
(f . i) `1 in Cn X by A4, A9, A10, A15;
hence (f . n) `1 in Cn X by ; :: thesis: verum
end;
suppose (f . n) `2 = 8 ; :: thesis: (f . n) `1 in Cn X
then consider i being Nat, p, q being Element of CQC-WFF Al, x being bound_QC-variable of Al such that
A17: 1 <= i and
A18: i < n and
A19: ( (f . i) `1 = p => q & not x in still_not-bound_in p & (f . n) `1 = p => (All (x,q)) ) by ;
i <= len f by ;
hence (f . n) `1 in Cn X by A4, A17, A18, A19, Th9; :: thesis: verum
end;
suppose (f . n) `2 = 9 ; :: thesis: (f . n) `1 in Cn X
then consider i being Nat, x, y being bound_QC-variable of Al, s being QC-formula of Al such that
A20: 1 <= i and
A21: i < n and
A22: ( s . x in CQC-WFF Al & s . y in CQC-WFF Al & not x in still_not-bound_in s & s . x = (f . i) `1 & (f . n) `1 = s . y ) by ;
i <= len f by ;
hence (f . n) `1 in Cn X by A4, A20, A21, A22, Th10; :: thesis: verum
end;
end;
end;
hence (f . n) `1 in Cn X ; :: thesis: verum
end;
for n being Nat holds S1[n] from hence for n being Nat st 1 <= n & n <= len f holds
(f . n) `1 in Cn X ; :: thesis: verum
end;
hence (f . l) `1 in Cn X by A2; :: thesis: verum