# Expression With Nested Refinements
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:
{% code overflow="wrap" %}
```
373873005 |Pharmaceutical / biologic product| :
411116001 |Has dose form| = ( 421720008 |Spray dose form| + 7946007 |Drug suspension|)
```
{% endcode %}
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:
```
24136001 |Hip joint structure| :
272741003 |Laterality| = 7771000 |Left|
```
This next expression uses the "left hip" expression above to describe a procedure to replace it:
```
397956004 |Prosthetic arthroplasty of the hip| :
363704007 |Procedure site| = ( 24136001 |Hip joint structure| :
272741003 |Laterality| = 7771000 |Left| )
```
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.
{% code overflow="wrap" %}
```
397956004 |Prosthetic arthroplasty of the hip| :
405814001 |Procedure site - indirect| = ( 24136001 |Hip joint structure| : 272741003 |Laterality| = 7771000 |Left| ),
{ 363699004 |Direct device| = 304120007 |Total hip replacement prosthesis| ,
260686004 |Method| = 425362007 |Surgical insertion - action| }
```
{% endcode %}
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.
```
243796009 |Situation with explicit context| :
{ 408730004 |Procedure context| = 385658003 |Done| ,
408731000 |Temporal context| = 410512000 |Current or specified| ,
408732007 |Subject relationship context| = 410604004 |Subject of record| ,
363589002 |Associated procedure| =
( 397956004 |Prosthetic arthroplasty of the hip| :
405814001 |Procedure site - indirect| = ( 24136001 |Hip joint structure| :
272741003 |Laterality| = 7771000 |Left| )
{ 363699004 |Direct device| = 304120007 |Total hip replacement prosthesis| ,
260686004 |Method| = 425362007 |Surgical insertion - action| } ) }
```
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