replace_program() - replaces the program in this_object()
void replace_program( string str );
replace_program() replaces the program in this_object() with that of an
object it inherits. The string argument is the filename of the object
whose program is to be used. Once the replacement takes place, the
current object effectively becomes a clone of that other object, but
with its current filename and global variable values. The program is
not actually replaced until the current execution is completed.