doublyevensubcode:=function(C)
 gen:=GeneratorMatrix(C);
 k:=Nrows(gen);
 if exists(singly){C!gen[i]:i in [1..k]|Weight(C!gen[i]) mod 4 eq 2} then
  return sub<C|[C!gen[i]-(Weight(C!gen[i]) div 2)*singly:i in [1..k]]>;
 else
  return C;
 end if;
end function;