Convert a rule structure into its head and body, or vice-versa. Note
that if an ID has been attached, it will be paired with the head as
Head = -(RealHead, ID). This can be taken advantage of if the head
and ID are simply being copied, but should be used with care.
ASP/Prolog operator table. Original table from the ISO Prolog
standard, with unsupported operators removed. NOTE: Some of the
operators below may not have been implemented yet.