Problem Statement

Write a function to flatten the nesting in an arbitrary list of values. Your program should work on the equivalent of this list: Where the correct result would be the list:

Let's start with the solution:

flatten(v)={
  my(u=[]);
  for(i=1,#v,
    u=concat(u,if(type(v[i])=="t_VEC",flatten(v[i]),v[i]))
  );
  u
};