When a program includes multiple functionalities, it is
constructed with two types of parts, one of which is “the
segment that is used only when a specific functionality is
executed (called Proper Part hereinafter), and “another
segment that is used when any functionality is executed
(Common Part hereinafter). If we take the meaning of the
proper part such that it is “the segment used only when a
specific functionality that we want to learn is executed”, then
we can understand that it must be “the segment that is
certainly implemented with such functionality that we want
to learn”.