The following sections are addressed to the predefined types supplied by SOS. They are defined in the schemas knl, agg, dir, and mta. The schema knl contains a minimal kernel of types which are necessary for any implementation; agg contains some predefined aggregate classes like e.g. List and Set that facilitate the definition of new (complex) classes and may be included by a user-defined schema. The schema dir provides a hierarchical directory facility resembling the UNIX file system. The schema mta defines the meta types, that means the types for the representation of types and their constituents, like e.g. methods.