let n be Nat; :: thesis: for f being FinSequence st len f <= n holds
f | n = f
let f be FinSequence; :: thesis: ( len f <= n implies f | n = f )
A1: dom f = Seg (len f) by FINSEQ_1:def 3;
assume len f <= n ; :: thesis: f | n = f
then ( f | n = f | (Seg n) & dom f c= Seg n ) by A1, FINSEQ_1:5;
hence f | n = f by RELAT_1:68; :: thesis: verum