In Modula 3, procedures are gathered together into modules. Sometimes a single module will perform all of the tasks required by the complete program. At other times a group of related procedures in a module will provide a set of useful procedures that may be used in more than one program - for instance a group of procedures that can be used on linked lists. In this case we will often call such modules a `library'. Modules and libraries are important because they allow us to encapsulate the effort and experience of previous problem solving projects and make future ones easier to solve. In the following examples we shall see that problems involving lists often use common procedures to simplify the overall task. We shall, therefore, assume a module of routines that are similar to those that you have already met in BE to solve some problems involving lists and sequences.