A single Scheme source file defines a library (although the file is likely to access other files). The last expression in the file must return a signed unit (see units) that imports the plt:userspace^ signature. When a user selects the library into DrScheme, DrScheme adds the names in the unit's export signature to the user's namespace.
The plt:userspace^ signature is defined in the userspcs.ss file of the userspce collection (see Library Collections and MzLib). A library file is always loaded with the MrEd language, but in case-sensitive mode (so that a library can export case-sensitive names to the teaching languages).
As an example example, the following signed unit defines a library that adds the binding four to the user's namespace:
(unit/sig (four) (import plt:userspace^) (define four 4))For more interesting examples, see the teach directory of the lib directory in the PLT installation.