specification.md

@@ -60,6 +60,7 @@ NDL model defines structure of the constraint programming problem and is compile
 
 NDL query is a Prolog like program, containing rules, with bodies consisting of goals from a predefined set of clauses:
 * NDL model clauses: `constant/2`, `variable/(2..6)`, `constraint/3`, `range_element/2`
+* negated clause `\+ constraint/3`, when at least on of the constraint arguments is already instantiated
 * 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