Jan Burse, created Aug 02. 2019
* Term building allows the access and construction of arbitrary terms.
* Since some of the predicates require arguments to be instantiated
* these predicates are not logically complete. Nevertheless most of
* the predicates are flexible enough so that they can be called in
* both directions. Namely they can be called with the modes (+, -),
* (-, +) and (+, +), whereby + indicates an instantiated argument.
* For performance reasons the interpreter performs unification without
* occurs check. This can result in cyclic structures which are not
* logically sound in the usual Herbrand model interpretation. The cyclic
* structures might result in infinitely looping programs or in a looping
* during term output. For programs that need a logically sound unification
* a special predicate is provided which does only instantiate variables
* when the check fails.
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* X = .. Y: [ISO 8.5.3]
* If X is atomic then the predicate succeeds when Y unifies with
* [X]. If X is the compound F(A1, .., An) then the predicate succeeds
* when Y unifies with [F, A1, …, An]. If Y is [C] and C is atomic
* then the predicate succeeds when X unifies with C. If Y is [F, A1, …, An]
* and F is an atom then the predicate succeeds when X unifies with F(A1, .., An).
% +-Term =.. -+List
sys_list_to_term/2, 'SpecialUniv', 6).
sys_term_to_list/2, 'SpecialUniv', 7).
* functor(X, N, A): [ISO 8.5.1]
* If X is atomic then the predicate succeeds when N unifies with X
* and A unifies with 0. If X is the compound f(A1, .., An) then the
* predicate succeeds when N unifies with f and A unifies with n. If N
* is atomic and A is 0 then the predicate succeeds when Y unifies
* with N. If N is an atom, A is an integer n≥1 and A1, …, An are
* fresh arguments then the predicate succeeds when Y unifies
* with N(A1, .., An).
% functor(+-Term, -+Atom, -+Integer)
% already defined in member
* arg(K, X, Y): [ISO 8.5.2]
* If K is a positive integer in the range of an arity and X is a
* callable f(A1, .., An) then the predicate succeeds when 1≤k≤n
* and Ak unifies with Y.
% arg(+Integer, +Term, -Term)
* set_arg(K, X, Y, Z):
* If K is a positive integer in the range of an arity and X is
* a callable f(A1, .., An) then the predicate succeeds when 1≤k≤n
* and Z unifies with f(A1, .., Ak-1, Y, Ak+1, .., An).
% set_arg(+Integer, +Term, +Term, -Term)
* X = Y: [ISO 8.2.1]
* The predicate succeeds when X and Y unify, no occurs check is performed.
% +Term = +Term
% already defined in member.p
* unify_with_occurs_check(X, Y): [ISO 8.2.2]
* The predicate succeeds when X and Y unify, occurs check is performed.
% unify_with_occurs_check(+Term, +Term)
unify_with_occurs_check/2, 'SpecialUniv', 4).
* X \= Y: [ISO 8.2.3]
* The predicate succeeds when X and Y do not unify, no occurs check is performed.
% +Term \= +Term