# cADL assertion, invariant, validity - any practical examples of use?

**URL:** <https://discourse.openehr.org/t/cadl-assertion-invariant-validity-any-practical-examples-of-use/14270>\
**Category:** Technical (archive)\
**Created:** [29 June 2009 17:10 UTC](https://discourse.openehr.org/t/cadl-assertion-invariant-validity-any-practical-examples-of-use/14270 "2009-06-29T17:10:51Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![Gavin\_Brelstaff](https://discourse.openehr.org/letter_avatar_proxy/v4/letter/g/e495f1/32.png) [@Gavin\_Brelstaff](https://discourse.openehr.org/u/Gavin_Brelstaff)\
**Post date:** [29 June 2009 17:10 UTC](https://discourse.openehr.org/t/cadl-assertion-invariant-validity-any-practical-examples-of-use/14270/1 "2009-06-29T17:10:51Z")

</div>

Hi

I'm considering how I might constrain layout in an EHR GUI depending  
on constraint made in an adl file.  
cADL assertions look a plausible way to go.  
But I don't see anyone using the invariant keyword in their ADL files  
- has anyone got any practical examples of doing so?

Any help gratefully accepted.

Gavin Brelstaff  
Sardinia

---

<div class="post-metadata">

**Author:** ![thomas.beale](https://discourse.openehr.org/user_avatar/discourse.openehr.org/thomas.beale/32/35_2.png) [@thomas.beale](https://discourse.openehr.org/u/thomas.beale)\
**Post date:** [30 June 2009 23:46 UTC](https://discourse.openehr.org/t/cadl-assertion-invariant-validity-any-practical-examples-of-use/14270/2 "2009-06-30T23:46:12Z")

</div>

gjb wrote: