code: public Tuple set(int idx, Object val) {
          while (idx >= fields.size()) {
              fields.add(null);
          }
          fields.set(idx, val);
          return this;
      }
docs: Sets a value at a specific position in the tuple.
  <p/>
  If this tuple has fewer elements than the index being set, then the tuple will grow to
  {@code idx} elements.
  
  @param idx the index to set
  @param val the value to set
  @return a handle to this object to enable builder operations