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The revised SNOMED CT Compositional Grammar allows attributes to be given concrete values (e.g. Strings, Integers, Decimal, Boolean). The SNOMED CT Expression Constraint Language supports the ability to compare these attribute values with a given concrete value.

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Concrete values of type string and boolean may also be included in an expression constraint, and compared using an 'equal to' (i.e. "=") or 'not equal to' (i.e. "!=") operator. The following expression constraint is satisfied only by products with a trade name equal to "PANADOL"

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	< 373873005 |Pharmaceutical / biologic product|:
		209999999104 |Has trade name| = "PANADOL"                

The following expression constraint is satisfied only by products that are in the national benefit scheme (of the given country).

	< 373873005 |Pharmaceutical / biologic product|:
		859999999102 |Is in national benefit scheme| = TRUE

Reverse Attributes

In most cases, an attribute refinement is satisfied by those concepts, which are the source concept of a defining relationship whose destination concept matches the attribute value. In some cases, however, it may be necessary to select the destination concept of a relationship and constrain the source concept to a given attribute value. To achieve this, an expression constraint indicates that an attribute is to be constrained in the reverse order using a 'reverse flag'

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