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Existence is one constraint statement that can be made using ADL/AOM about attributes in a reference model. In ADL as statement will look something like:

ELEMENT matches
{
   value existence matches {1} matches*
}

This indicates that the value attribute has to be present. In fact, to this point such statements do not occur in ADL; only statements that an attributes existence is optional occur.

ELEMENT matches
{
   value existence matches {0..1} matches*
}

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