expansion of List variables inside List variables not working

This is a reproducer of the problem:

from xenv.Control import Environment
env = Environment()

env.set('A_PATH', '/usr/bin:/bin')
assert str(env['A_PATH'])

env.set('B_PATH', '${A_PATH}')
assert str(env['B_PATH'])  # fails because the directory '/usr/bin:/bin' does not exist

assert str(env['A_PATH']) == str(env['B_PATH'])