This reverts commit c5bea524.

This reverts commit b18f6cc1.

Improve constructors.

    - Use (const UList<token>&) instead of (const List<token>&)

    - Add (const Xfer< List<token> >&) constructor.

- can use 'XX.empty()' instead of 'XX.size() == 0', 'XX.size() < 1' or
  'XX.size() <= 0' or for simpler coding.
  It also has the same number of characters as '!XX.size()' and /might/ be
  more readable

- many size checking had 'XX.size() > 0', 'XX.size() != 0', or 'XX.size() >= 1'
  when a simple 'XX.size()' suffices