let i be Element of NAT ; :: thesis: for a being set
for p, q being FinSequence st p +* i,a = q +* i,a holds
Del p,i = Del q,i
let a be set ; :: thesis: for p, q being FinSequence st p +* i,a = q +* i,a holds
Del p,i = Del q,i
let p, q be FinSequence; :: thesis: ( p +* i,a = q +* i,a implies Del p,i = Del q,i )
set x = p;
set y = q;
assume A1: p +* i,a = q +* i,a ; :: thesis: Del p,i = Del q,i
set xi = p +* i,a;
set yi = q +* i,a;
set dx = Del p,i;
set dy = Del q,i;
A2: dom (p +* i,a) = dom p by FUNCT_7:32;
A3: dom (q +* i,a) = dom q by FUNCT_7:32;
A4: Seg (len q) = dom q by FINSEQ_1:def 3;
A5: Seg (len p) = dom p by FINSEQ_1:def 3;

per cases ( i in dom p or not i in dom p ) ;

suppose A6: i in dom p ; :: thesis: Del p,i = Del q,i

A7: dom (Del p,i) = Seg (len (Del p,i)) by FINSEQ_1:def 3;

now

thus len (Del p,i) = len (Del p,i) ; :: thesis: ( len (Del q,i) = len (Del p,i) & ( for j being Nat st j in dom (Del p,i) holds
(Del p,i) . b₂ = (Del q,i) . b₂ ) )
p <> {} by A6;
then consider m being Nat such that
A8: len p = m + 1 by NAT_1:6;
reconsider m = m as Element of NAT by ORDINAL1:def 13;
A10: len (Del p,i) = m by A6, A8, FINSEQ_3:118;
hence len (Del q,i) = len (Del p,i) by A1, A2, A3, A5, A4, A6, A8, FINSEQ_1:8, FINSEQ_3:118; :: thesis: for j being Nat st j in dom (Del p,i) holds
(Del p,i) . b₂ = (Del q,i) . b₂
let j be Nat; :: thesis: ( j in dom (Del p,i) implies (Del p,i) . b₁ = (Del q,i) . b₁ )
assume j in dom (Del p,i) ; :: thesis: (Del p,i) . b₁ = (Del q,i) . b₁
then A11: j <= m by A7, A10, FINSEQ_1:3;

per cases ( j < i or i <= j ) ;

suppose A13: j < i ; :: thesis: (Del p,i) . b₁ = (Del q,i) . b₁

hence (Del p,i) . j = p . j by FINSEQ_3:119
.= (q +* i,a) . j by A1, A13, FUNCT_7:34
.= q . j by A13, FUNCT_7:34
.= (Del q,i) . j by A13, FINSEQ_3:119 ;
:: thesis: verum

end;

suppose A14: i <= j ; :: thesis: (Del p,i) . b₁ = (Del q,i) . b₁

then A15: j + 1 > i by NAT_1:13;
thus (Del p,i) . j = p . (j + 1) by A6, A8, A11, A14, FINSEQ_3:120
.= (q +* i,a) . (j + 1) by A1, A15, FUNCT_7:34
.= q . (j + 1) by A15, FUNCT_7:34
.= (Del q,i) . j by A1, A2, A3, A5, A4, A6, A8, A11, A14, FINSEQ_1:8, FINSEQ_3:120 ; :: thesis: verum

end;

end;

end;

hence Del p,i = Del q,i by FINSEQ_2:10; :: thesis: verum

end;

suppose A16: not i in dom p ; :: thesis: Del p,i = Del q,i

then p +* i,a = p by FUNCT_7:def 3;
hence Del p,i = Del q,i by A1, A3, A16, FUNCT_7:def 3; :: thesis: verum

end;

end;