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@@ -103,6 +103,43 @@ For associations, we have the same three attributes, augmented with:
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6. *target_lower_cardinality* specifies the number (integer) of outgoing edges that are at least required for the source.
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7. *target_upper_cardinality* specifies the number (integer) of outgoing edges that are at most required for the source.
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+Constraints
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+^^^^^^^^^^^
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+
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+A constraint specifies additional constraints on the instances of this model, which could not be expressed before.
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+We distinguish between two types of constraints: local and global constraints.
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+
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+Local constraints are defined at the level of a single class or association.
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+For example, if the number of tokens must always be non-negative, this can be specified as follows::
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+
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+ >>> attr_assign_code("PetriNets", "Place", "constraints", \
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+ ... """
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+ ... String function constraint(model : Element, name : String):
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+ ... if (integer_gte(read_attribute(model, name, "tokens"), 0)):
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+ ... return "OK"!
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+ ... else:
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+ ... return "Negative number of tokens specified"!
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+
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+Global constraints operate over the complete model, and are therefore not attached to a single element.
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+Instead, a specific *GlobalConstraint* element must be instantiated.
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+This element has a *constraint* attribute, which specifies the constraint to evaluate in the global context.
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+For example, when, for some reason, the number of places must always be larger than the number of transitions::
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+
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+ >>> constraint = instantiate("PetriNets", "GlobalConstraint")
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+ >>> attr_assign_code("PetriNets", constraint, "constraint", \
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+ ... """
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+ ... String function constraint(model : Element):
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+ ... if (set_len(allInstances(model, "Place")) > set_len(allInstances(model, "Transition"))):
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+ ... return "OK"!
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+ ... else:
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+ ... return "Needs more places than transitions"!
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+
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+While all constraints can be specified as global constraints, using local constraints is preferred.
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+
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+.. note::
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+ Currently, all constraints are only evaluated upon explicit user request.
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+ More specifically: during the *verify* operation.
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+
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Defining attributes
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^^^^^^^^^^^^^^^^^^^
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@@ -145,9 +182,3 @@ This can be easily done as follows::
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>>> define_attribute("PetriNets", "Place", "tokens", "Natural")
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For the name, *String* can be defined similarly (using *is_physical_string*).
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-
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-Constraints
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-^^^^^^^^^^^
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-
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-A constraint specifies additional constraints on the instances of this model, which could not be expressed before.
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-We distinguish between two types of constraints:
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