So far, all of this book's example queries were contained in one module, known as the main module. However, you can declare functions and variables in other modules and import them into the main module of the query. This is a very useful feature for:
Reusing functions among many queries
Creating standardized libraries that can be distributed to a variety of query users
Organizing and reducing the size of query modules
The main module contains a query prolog followed by a query body, which is the main expression to be evaluated. In its prolog, the main module can import other modules known as library modules.
Not all implementations support the use of library modules; it is an optional feature.
Library modules differ from main modules in that they cannot have a query body, only a prolog. They also differ in that they must start with a module declaration, whose syntax is shown in Figure 12-2.
Figure 12-2. Syntax of a module declaration
The module declaration identifies the module as a library module. It also declares the target namespace of the module and maps it to a prefix. For example, the expression:
module namespace strings = "http://datypic.com/strings";
declares the target namespace of the module to be http://datypic.com/strings and binds that namespace to the prefix
The target namespace must be a literal value in quotes, ...