specification.md

@@ -59,12 +59,25 @@ NDL model defines structure of the constraint programming problem and is compile
 ## Query 
 
 NDL query is a Prolog like program, containing rules, with bodies consisting of goals from a predefined set of clauses:
-* NDL model clauses: `constraint/2`, `constraint_range/2`, `variable/2`, `new_value/2`, `constraint/3`, `is_satisfied/3`
-* binary arithmetic operations: +, -, /, *, div, mod
-* binary arithmetic comparisons: =, >=, >, <, <=
-* heads of the rules defined in the query itself (allows recursion)
+* NDL model clauses: `constant/2`, `variable/(2..6)`, `constraint/3`, `range_element/2`
+* binary arithmetic operations: `+/2`, `//2`, `-/2`, `*/2`, `mod/2`, `min/2`, `max/2`, `abs/1`
+  * uses Prolog `is` notation
+  * only binary and unary expressions are allowed
+  * dividing by `0` gives `0` 
+  * at least one of the arguments has to be of type `range`, constants are promoted to `ranges` automatically
+* binary arithmetic comparisons: `=/2`, `>=/2`, `>/2`, `</2`, `<=/2`
+* loops in form of `for_each(+Generator:model_clause, +Query:query)`
+  * first argument is a non-deterministic model clause, i.e. `variable/(2..6)`, `constraint/3`, `range_element/2`
+  * `Generator`'s variables are available in the `Query`
+  * queries are run in order, independently, it the query fails on the specific variable assignment, it continues with the next assignment
+* 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
+  * if `Condition` fails and `Else` query is provided, then `Else` is evaluated
+  * if `Condition` fails and `Else` query is not provided, then nothing happens and expression succeeds (non-standard in the Prolog world)
 * operation operating on the current solution:
   * `get_value(+Index:variable_index, -Value:integer)` - gets the variable's current value
   * `set_value(+Index:variable_index, +Value:integer)` - set the variable's value to `Value`
   * `swap_values(+Index1:variable_index, +Index2:variable_index)` - swap values of two variables
+  * `flip_variable(+Index:variable_index, +HeadsValue:integer, +TailsValue:integer)` - if variable has value `HeadsValue` or `TailsValue`, sets the value to the other one. Otherwise fails.