The syntax of CHR rules is the following:
| rules | ::= rule rules
|
| rules | ::= empty
|
| rule | ::= name actual_rule pragma .
|
| name | ::= atom @
|
| name | ::= empty
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| actual_rule | ::= simplification_rule
|
| actual_rule | ::= propagation_rule
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| actual_rule | ::= simpagation_rule
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| simplification_rule | ::= head <=> guard body
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| propagation_rule | ::= head ==> guard body
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| simpagation_rule | ::= head \ head <=> guard body
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| head | ::= constraints
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| constraints | ::= constraint constraint_id
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| constraints | ::= constraint constraint_id , constraints
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| constraint | ::= compound_term
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| constraint_id | ::= empty
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| constraint_id | ::= # variable
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| guard | ::= empty
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| guard | ::= goal |
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| body | ::= goal
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| pragma | ::= empty
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| pragma | ::= pragma actual_pragmas
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| actual_pragmas | ::= actual_pragma
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| actual_pragmas | ::= actual_pragma , actual_pragmas
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| actual_pragma | ::= passive(variable)
|
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.