The interface for IntSortedList as specified
in the exercise sheet.
Implement+test TreeIntSortedList... Test output->
... storing and writing out an ordered list in a tree
is really efficient!

-> Test output shows that everything works well list output in order ...
s$ java TestIntSortedList
Run tests on TreeIntSortedList:
add 7 to the list
add 9 to the list
add 7 to the list again!
print the list contents in order: 7, 7, 9
verbose test whether list contains 9:
	Result list does contain 9
extend list and print in order 1, 3, 7, 7, 8, 9, 9, 13, 13, 14, 18, 19, 21, 22, 22, 22, 23, 26, 27, 29, 29, 33, 34, 34, 35, 37, 37, 39, 41, 43, 44, 45, 49, 50, 50
verbose test whether list contains 19:
	Result list does contain 19
verbose test whether list contains 28:
	Result list does not contain 28
write up 3.1 and 3.2