Exercise 3

modify DemoBuiltins and DemoStrings:
change all variable names to something different, but leave the
rest of the program intact.

verify the program still compiles and has the same output
after your modification!

---

this is an exercise on syntax too: the idea is to quickly see
what are tokens of the Java language and library and what are
tokens the programmer defines

note: consistently changing the name of variable in different parts
of a program is called refactoring - modern IDEs can do this
automatically