specification.md

@@ -71,6 +71,12 @@ NDL query is a Prolog like program, containing rules, with bodies consisting of
   * first argument is a non-deterministic model clause, i.e. `variable/(2..6)`, `constraint/3`, `range_element/2`
   * all `Generator`'s variables are instantiated and available in the `Query`
   * queries are run in order, independently, it the query fails on the specific variables' instantiation, it continues with the next assignment
+* breadth-first search walk over the constraint graph, in form `walk_over(+Constraint:constraint_query, +Start:constrained_element, +Query:query)`, where:
+  * `Constraint` is a model clause `constraint/3` with no bound arguments
+  * `Start` defines the point of walk start; it should be a value that appears as the first argument of Constraint in the model (there is a second operation `walk_over_inverted/3` that inverts the edges direction)
+  * `Query` can use variables introduced in the `Constraint` term
+  * if `Query` fails during the walk, the failed branch gets cut, but the walk continues
+  * BFS remembers all the visited edges, so no edge is visited twice 
 * conditions `if(+Condition:query,+Then:query)` and `if(+Condition:test,+Then:query,+Else:query)`
   * `Condition` contains only one term
   * if `Condition` succeeds, `Then` query is evaluated