return current char and step if char is between lower and upper, where 
   @test: a python function with one argument, which tests on one char and return True or False
          @test must be registered with register_function

def char_between(lower, upper, func_name): 
  '''return current char and step if char is between lower and upper, where 
   @test: a python function with one argument, which tests on one char and return True or False
          @test must be registered with register_function'''
  function = register_function(func_name, 
                      lambda char: lower<=char<=upper)
  return char_on_predicate(function)