deffunc H₁( Nat) -> Element of NAT = IFGT ($1,k,0,1);
consider f being sequence of NAT such that
A1: 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 = IFGT (n,k,0,1)
let n be Nat; :: thesis: f . n = IFGT (n,k,0,1)
n in NAT by ORDINAL1:def 12;
hence f . n = IFGT (n,k,0,1) by A1; :: thesis: verum