aggregate.pl  Aggregation operators on backtrackable predicates
This library provides aggregating operators over the solutions of a predicate. The operations are a generalisation of the bagof/3, setof/3 and findall/3 builtin predicates. Aggregations that can be computed incrementally avoid findall/3 and run in constant memory. The defined aggregation operations are counting, computing the sum, minimum, maximum, a bag of solutions and a set of solutions. We first give a simple example, computing the country with the smallest area:
smallest_country(Name, Area) : aggregate(min(A, N), country(N, A), min(Area, Name)).
There are four aggregation predicates (aggregate/3, aggregate/4, aggregate_all/3 and aggregate/4), distinguished on two properties.
 aggregate vs. aggregate_all

The aggregate predicates use setof/3 (aggregate/4) or bagof/3
(aggregate/3), dealing with existential qualified variables
(
Var^Goal
) and providing multiple solutions for the remaining free variables in Goal. The aggregate_all/3 predicate uses findall/3, implicitly qualifying all free variables and providing exactly one solution, while aggregate_all/4 uses sort/2 over solutions that Discriminator (see below) generated using findall/3.  The Discriminator argument

The versions with 4 arguments deduplicate redundant solutions of
Goal. Solutions for which both the template variables and
Discriminator are identical will be treated as one solution. For
example, if we wish to compute the total population of all
countries, and for some reason
country(belgium, 11000000)
may succeed twice, we can use the following to avoid counting the population of Belgium twice:aggregate(sum(P), Name, country(Name, P), Total)
All aggregation predicates support the following operators below in
Template. In addition, they allow for an arbitrary named compound term,
where each of the arguments is a term from the list below. For example,
the term r(min(X), max(X))
computes both the minimum and maximum binding
for X.
 count
 Count number of solutions. Same as
sum(1)
.  sum(Expr)
 Sum of Expr for all solutions.
 min(Expr)
 Minimum of Expr for all solutions.
 min(Expr, Witness)
 A term
min(Min, Witness)
, where Min is the minimal version of Expr over all solutions, and Witness is any other template applied to solutions that produced Min. If multiple solutions provide the same minimum, Witness corresponds to the first solution.  max(Expr)
 Maximum of Expr for all solutions.
 max(Expr, Witness)
 As
min(Expr, Witness)
, but producing the maximum result.  set(X)
 An ordered set with all solutions for X.
 bag(X)
 A list of all solutions for X.
Acknowledgements
The development of this library was sponsored by SecuritEase, http://www.securitease.com
 aggregate(+Template, :Goal, Result) is nondet
 Aggregate bindings in Goal according to Template. The aggregate/3 version performs bagof/3 on Goal.
 aggregate(+Template, +Discriminator, :Goal, Result) is nondet
 Aggregate bindings in Goal according to Template. The aggregate/4 version performs setof/3 on Goal.
 aggregate_all(+Template, :Goal, Result) is semidet
 Aggregate bindings in Goal according to Template. The
aggregate_all/3 version performs findall/3 on Goal. Note that this
predicate fails if Template contains one or more of
min(X)
,max(X)
,min(X,Witness)
ormax(X,Witness)
and Goal has no solutions, i.e., the minimum and maximum of an empty set is undefined.The Template values
count
,sum(X)
,max(X)
,min(X)
,max(X,W)
andmin(X,W)
are processed incrementally rather than using findall/3 and run in constant memory.  aggregate_all(+Template, +Discriminator, :Goal, Result) is semidet
 Aggregate bindings in Goal according to Template. The aggregate_all/4 version performs findall/3 followed by sort/2 on Goal. See aggregate_all/3 to understand why this predicate can fail.
 foreach(:Generator, :Goal)
 True if conjunction of results is true. Unlike forall/2, which
runs a failuredriven loop that proves Goal for each solution of
Generator, foreach/2 creates a conjunction. Each member of the
conjunction is a copy of Goal, where the variables it shares
with Generator are filled with the values from the corresponding
solution.
The implementation executes forall/2 if Goal does not contain any variables that are not shared with Generator.
Here is an example:
? foreach(between(1,4,X), dif(X,Y)), Y = 5. Y = 5. ? foreach(between(1,4,X), dif(X,Y)), Y = 3. false.
 free_variables(:Generator, +Template, +VarList0, VarList) is det
 Find free variables in bagof/setof template. In order to handle
variables properly, we have to find all the universally
quantified variables in the Generator. All variables as yet
unbound are universally quantified, unless
free_variables(Generator, Template, OldList, NewList)
finds this set using OldList as an accumulator.  sandbox:safe_meta(+Goal, Called) is semidet[multifile]
 Declare the aggregate metacalls safe. This cannot be proven due to the manipulations of the argument Goal.