specification.md

@@ -87,6 +87,7 @@ There are several extensions to the NDL query, that can be used in order to incr
 
 ### Stochastic mode
 
+Motivation: many meta-heuristics sample only a random subset of neighbohood.
 NDL Queries can be compiled as stochastic operator. The stochastic mode impacts ordering of the `variable/(2..6)`, `constraint/3`, `range_element/2` results, effectively returning them in a random fashion. It does **not** impact `for_each` generator.
 
 **Warning:** stochastic ndl query always backtracks.
@@ -99,4 +100,15 @@ Motivation: often one would like to mark a variable as already moved, so the que
 * `in_memory(+Arg)` - checks if the argument has ben remembered
 * `\+ in_memory(+Arg)` - succeeds if the argument hasn't been remembered
 
+### Stack
+
+Motivation: stack is a useful data structure, allowing to simulate primitive recursion.
+Comparing to the **Memory** extension, stacks are ordered (LIFO) and can be queried.
+This extension adds three additional constructs:
+
+* `push(+Type:type, +Arg:Type)` - pushes value of `Type` on the stack
+* 'pop(+Type:type, -Arg:Type)` - pops value of `Type` from the stack, fails if there is no value of this type on the stack
+* `top(+Type:type, -Arg:Type)` - gets the last added element of `Type` from the stack, but doesn't remove it from the stack
+* new loop `while(pop(+Type:type, -Arg:Type), Query:query)` - that loops while the stack contains value of a type `Type`
+