deffunc H₁( Nat) -> Element of K10(X) = (A1 . $1) \ A;
consider f being SetSequence of X such that
A10: for n being Element of NAT holds f . n = H₁(n) from FUNCT_2:sch 4();
take f ; :: thesis: for n being Nat holds f . n = (A1 . n) \ A
let n be Nat; :: thesis: f . n = (A1 . n) \ A
n in NAT by ORDINAL1:def 12;
then f . n = H₁(n) by A10;
hence f . n = (A1 . n) \ A ; :: thesis: verum