specification.md

@@ -112,7 +112,7 @@ This extension adds three additional constructs:
 * `top(+Type:type, -Arg:Type)` - gets the last added element of `Type` from the stack, but doesn't remove it from the stack
 * `while(pop(+Type:type, -Arg:Type), Query:query)` - new loop that loops while the stack contains value of a type `Type`. It can be used to simulate recursion, there is no guarantee it finishes.
 
-## Semantics
+### Semantics
 
 Motivation: user can define constraint and auxiliary variables semantics in the model. Exploiting semantic part of the model often simplifies the operator.
 This extension adds additional constructs:
@@ -121,19 +121,30 @@ This extension adds additional constructs:
 * `is_satisfied(+Name:name, +Arg1:arg1(Name), +Arg2:arg2(Name))` - succeeds if the constraint named `Name` is satisfied for arguments `Arg1` and `Arg2`
 * `is_violated(+Name:name, +Arg1:arg1(Name), +Arg2:arg2(Name))` - succeeds if the constraint named `Name` is not satisfied for arguments `Arg1` and `Arg2`
 
-## Generative semantics
+### Generative semantics
 
 Motivation: the common strategy when implementing a neighborhood operator is to make a move and then fix constraints that got violated by the move. 
 
 The biggest difference between the **normal** and *generative* semantics is that in the latter case one can query the violated constraints in a generative manner.
 This extension adds additional constructs:
 
-* all from the weak semantics
+* all from the normal semantics
 * `violated(+Name:name, ?Arg1:arg1(Name), ?Arg2:arg2(Name)) is nondet` - finds a violated constraint named `Name` if there is any. Otherwise it fails.
 * loop `for_each(violated(+Name:name, ?Arg1:arg1(Name), ?Arg2:arg2(Name)), +Query:query)` 
 * loop `while(violated(+Name:name, ?Arg1:arg1(Name), ?Arg2:arg2(Name)), +Query:query)` - a common patter used in the neighborhood operators.
 
 **Warning** using generative semantics adds an overhead to all operations modifying the solution.
  
+### Custom predicates 
+
+Motivation: it's often desirable to group common query parts into named predicates.
+This extension allows to define custom (possibly recursive) predicates that can be used later in the query. 
+Every predicate has to have defined name, arity and types of its' arguments, i.e. `predicate/2 : [arg1type, arg2type]`. 
+Using this extension it is possible to replace all control structure (loops, conditional, stacks, memory) with recursion, making it a plain logic program suitable for logical induction approaches.
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