This chapter is from the book
Inviting Message
Sometimes as you are writing code, you expect that people will want to vary a part of the computation in a subclass. Send an appropriately named message to communicate the possibility of later refinement. The message invites programmers to refine the computation for their own purposes later.
If there is a default implementation of the logic, make it the implementation of the message. If not, declare the method abstract to make the invitation explicit.