This is an assignment that will be run through in class, so this time
the deadline is strict.

But no need for panic. It is really a 218 assignment. Its purpose is
to make sure everyone remembers the basics before it is too late, and
to give us an opportunity to look back and see how we could do better.

Write a program in C++ that:

1. accepts an unlimited number of strings (none of which will contain spaces)
   typed by the user

2. adds those strings, as they are typed, into an initially empty binary tree

     The order of the tree is alphabetical, and case INsensitive, so cat and CAT
     are the same string, and ant comes before Zebra.

3. when the user signals that the data entry phase is over, the program must then
   accept another unlimited sequence of strings from the user

4. when each string is typed, the program searches the tree for it, and prints
   "yes" if it is present, or "no" if it isn't.