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It is also possible to nest expressions, one inside the other. Any valid expression may be enclosed in a pair of brackets, and included as the value of an attribute in another expression. For example, the following expression describes a medication product that has a single dose form, which is both a spray and a suspension:

In the example above, note the use of round brackets (i.e. "( )") to identify a nested expression, as opposed to braces (i.e. "{ }"), which is used to identify attribute groups.

The following examples show how complex expressions may be build up from simple ones, a layer at a time. This first expression describes a left hip structure:

This next expression uses the "left hip" expression above to describe a procedure to replace it: Applying a further grouped refinement to the above describes a procedure to replace a left hip by inserting a prosthesis. Note that this example mixes an ungrouped qualification and a grouped qualification. Where this is done, the canonical representation of the expression includes all ungrouped attributes before the grouped ones (see  Terminology Services Guide for more details). Please also note that in the previous version of this standard a comma was not permitted between the last ungrouped attribute and the first attribute group – however this comma is now optional to avoid unexpected syntax errors. Finally, the above expression may be included within a contextual wrapper, to describe a procedure that has been performed on a patient to replace a left hip by inserting a prosthesis.