Nested HEX-Programs
  Answer-Set Programming (ASP) is an established declarative programming
paradigm. However, classical ASP lacks subprogram calls as in procedural
programming, and access to external computations (like remote procedure calls)
in general. The feature is desired for increasing modularity and---assuming
proper access in place---(meta-)reasoning over subprogram results. While
HEX-programs extend classical ASP with external source access, they do not
support calls of (sub-)programs upfront. We present nested HEX-programs, which
extend HEX-programs to serve the desired feature, in a user-friendly manner.
Notably, the answer sets of called sub-programs can be individually accessed.
This is particularly useful for applications that need to reason over answer
sets like belief set merging, user-defined aggregate functions, or preferences
of answer sets.
