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

**Category:** [Technical (archive)](https://discourse.openehr.org/c/technical-archive/156)
**Created:** 2009-06-29 17:10 UTC
**Views:** 1
**Replies:** 1
**URL:** https://discourse.openehr.org/t/cadl-assertion-invariant-validity-any-practical-examples-of-use/14270

---

## Post #1 by @Gavin_Brelstaff

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

---

## Post #2 by @thomas.beale

gjb wrote:

---


**Canonical:** https://discourse.openehr.org/t/cadl-assertion-invariant-validity-any-practical-examples-of-use/14270
**Original content:** https://discourse.openehr.org/t/cadl-assertion-invariant-validity-any-practical-examples-of-use/14270