let D be non empty set ; :: thesis: for p1, p2, p3, q being Element of D holds Replace <*p1,p2,p3*>,2,q = <*p1,q,p3*>
let p1, p2, p3, q be Element of D; :: thesis: Replace <*p1,p2,p3*>,2,q = <*p1,q,p3*>
set f = <*p1,p2,p3*>;
A1: len <*p1,p2,p3*> = 3 by FINSEQ_1:62;
then A2: 3 in dom <*p1,p2,p3*> by FINSEQ_3:27;
A3: 1 in dom <*p1,p2,p3*> by A1, FINSEQ_3:27;
A4: 2 -' 1 = (1 + 1) -' 1
.= 1 by NAT_D:34 ;
A5: len <*p1,p2,p3*> = 2 + 1 by FINSEQ_1:62;
Replace <*p1,p2,p3*>,2,q = ((<*p1,p2,p3*> | (2 -' 1)) ^ <*q*>) ^ (<*p1,p2,p3*> /^ 2) by A1, Def1
.= ((<*p1,p2,p3*> | 1) ^ <*q*>) ^ <*(<*p1,p2,p3*> /. 3)*> by A4, A5, FINSEQ_5:33
.= ((<*p1,p2,p3*> | 1) ^ <*q*>) ^ <*(<*p1,p2,p3*> . 3)*> by A2, PARTFUN1:def 8
.= ((<*p1,p2,p3*> | 1) ^ <*q*>) ^ <*p3*> by FINSEQ_1:62
.= (<*(<*p1,p2,p3*> /. 1)*> ^ <*q*>) ^ <*p3*> by FINSEQ_5:23
.= (<*(<*p1,p2,p3*> . 1)*> ^ <*q*>) ^ <*p3*> by A3, PARTFUN1:def 8
.= (<*p1*> ^ <*q*>) ^ <*p3*> by FINSEQ_1:62 ;
hence Replace <*p1,p2,p3*>,2,q = <*p1,q,p3*> ; :: thesis: verum