todo expression
---------------
Behaves like the "undefined" value. Optionally prints an error message
and/or terminates the program (depending on which command line options
were given to the compiler) when the keyword is used in an expression.

    int i;
    i = todo; // error at runtime! "assigns" undefined to i
    
    i.print(); // error and/or prints garbage (i has an undefined value)

The todo expression can be used as a statement, like this:

    () f(bool x, bool y) {
        if (x) todo; // error at runtime!
        
        if (y) {
            "y is true".print()
            todo; // error at runtime!
        }
    }
    
    () g() {
        todo; // error at runtime!
    }


todo in loops
-------------

    for (int i in todo) { // error at runtime! behaves like []
        i.print();  // not executed
    }


todo types
----------
Behaves like a private / undefined type.

    typedef A = todo;