Jan Burse, created Aug 17. 2019
* The test predicate ground/1 checks whether the given term is
* ground. This means that no un-instantiated variable occurs in
* the term. The predicate term_variables/2 allows collecting the
* un-instantiated variables that occur in a term. The predicate
* will thus return an empty list if the term was ground. Finally
* the predicate sys_term_singletons/2 collects the un-instantiated
* variables that only occur once. They are a subset of all the
* variables that occur in the term.
* ?- sys_goal_kernel(X^p(X,Y),K).
* K = p(X,Y)
* ?- sys_goal_globals(X^p(X,Y),L).
* L = [Y]
* Further there are predicates to deal with existential quantifiers.
* The existential quantifier is represented by the (^)/2 operator.
* In a goal X1^..^Xn^K we call K the kernel of the goal and the
* variables K subtracted by the variables of X1,..,Xn the global
* variables of the goal. The predicates sys_goal_kernel/2 and
* sys_goal_globals/2 cater for the determination of the kernel
* and the global variables of a goal.
* An alternative to using the ‘$VAR’() construct is dynamically
* creating a variable names map. This has the advantage that the construct
* itself can be written out. The predicate sys_number_variables/4 helps
* in creating a variable names map. The resulting variable names map
* can be used with the predicates write_term/[2,3].
* The variable names map from the current top-level query can be
* retrieved via the predicate sys_get_variable_names/1. The predicate
* will skip non-variable and duplicate entries. In the case of duplicates
* the entry with a lower dereferencing count is preferred. The result is
* intended to be used with the predicates write_term/[2,3].
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* term_variables(X, L): [TC2 8.5.5]
* term_variables(X, L, R):
* The predicate succeeds when L unifies with the variables of X.
* The ternary variant produces a difference list.
% term_variables(+Term, -List)
% term_variables(+Term, -List, +List)
term_variables/3, 'SpecialVars', 0).
* sys_term_singletons(X, L):
* The predicate succeeds when L unifies with the variables of X
* that occur only once.
% sys_term_singletons(+Term, -List)
sys_term_singletons/2, 'SpecialVars', 1).
* sys_goal_kernel(G, K):
* The predicate succeeds when K unifies with the kernel of the goal G.
% sys_goal_kernel(+GoalQuant, -Goal)
sys_goal_kernel/2, 'SpecialVars', 2).
* sys_goal_globals(G, L):
* The predicate succeeds when L unifies with the global variables of the goal G.
% sys_goal_globals(+GoalQuant, -List)
sys_goal_globals/2, 'SpecialVars', 3).
* numbervars(X, N, M):
* The predicate instantiates the un-instantiated variables of the term X with
* compounds of the form ‘$VAR’(). The starts with N. The predicate
* succeeds when M unifies with the next available .
% numbervars(+Term, +Integer, -Integer)
* sys_number_variables(V, N, S, M):
* The predicate succeeds with variable names M resulting from giving names
* to the variables in V, respecting the variable names N and the unnamed
* singletons S.
% sys_number_variables(+List, +Assoc, +List, -Assoc)
sys_number_variables/4, 'SpecialVars', 5).
* ground(X): [TC2 8.3.10]
* The predicate succeeds when X is a ground term, i.e. contains
* no variables.
% already defined in member
* acyclic_term(X): [TC2 8.3.11]
* The predicate succeeds when X is an acyclic term, i.e. contains
* no cycles.
acyclic_term/1, 'SpecialVars', 6).
* safe_term_variables(X, L):
* safe_term_variables(X, L, R):
* The predicate succeeds when L unifies with the variables of X,
* even when X is a cyclic term. The ternary variant produces a
* difference list.
% safe_term_variables(+Term, -List)
% safe_term_variables(+Term, -List, +List)
safe_term_variables/3, 'SpecialVars', 7).