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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:
373873005 |pharmaceutical / biologic product|:
411116001|has dose form| =
(421720008|spray dose form| + 7946007 |drug 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:
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 (section 7.8.2.4.4.4.2 of the SNOMED CT Technical Implementation Guide [6]). 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.
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|}
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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