ATSLIB is a set of libraries included with ATS, which seems to include all libraries except those included in [[contrib|contrib]] and a few others that are primarily designed to be used by ATS itself, such as libatslex.
ATSLIB currently includes prelude, libc, and libats (see codegen/Makefile_atslib for how ATSLIB is built).
Examples using ATSLIB can be found in doc/EXAMPLE/ATSLIB.