code: public final long set0(int idx, long l) {
      setWrite();
      if( _chk2.set_impl(idx,l) ) return l;
      (_chk2 = inflate_impl(new NewChunk(this))).set_impl(idx,l);
      return l;
    }
docs: Set a long element in a chunk given a 0-based chunk local index.
  
  Write into a chunk.
  May rewrite/replace chunks if the chunk needs to be
  "inflated" to hold larger values.  Returns the input value.
  
  Note that the idx is an int (instead of a long), which tells you
  that index 0 is the first row in the chunk, not the whole Vec.