reserve i,n,m,k,x for Nat,
  i1,i2 for Integer;
reserve a for Tuple of n,k-SD;
reserve aSub for Tuple of n,k-SD_Sub;

theorem
  i in Seg n implies DigA_SDSub(aSub,i) is Element of k-SD_Sub
proof
  assume
A1: i in Seg n;
  then aSub.i = DigA_SDSub(aSub,i) by Def5;
  hence thesis by A1,Th11;
end;
