9.2.1 Syntax of CHR rules

rules --> rule, rules ; [].

rule --> name, actual_rule, pragma, [atom('.')].

name --> atom, [atom('@')] ; [].

actual_rule --> simplification_rule.
actual_rule --> propagation_rule.
actual_rule --> simpagation_rule.

simplification_rule --> head, [atom('<=>')], guard, body.
propagation_rule --> head, [atom('==>')], guard, body.
simpagation_rule --> head, [atom('\')], head, [atom('<=>')],
                     guard, body.

head --> constraints.

constraints --> constraint, constraint_id.
constraints --> constraint, constraint_id,
                [atom(',')], constraints.

constraint --> compound_term.

constraint_id --> [].
constraint_id --> [atom('#')], variable.
constraint_id --> [atom('#')], [atom('passive')] .

guard --> [] ; goal, [atom('|')].

body --> goal.

pragma --> [].
pragma --> [atom('pragma')], actual_pragmas.

actual_pragmas --> actual_pragma.
actual_pragmas --> actual_pragma, [atom(',')], actual_pragmas.

actual_pragma --> [atom('passive(')], variable, [atom(')')].

Note that the guard of a rule may not contain any goal that binds a variable in the head of the rule with a non-variable or with another variable in the head of the rule. It may, however, bind variables that do not appear in the head of the rule, e.g. an auxiliary variable introduced in the guard.