Implementation of targetConstraint extension

[samturneradha]

Is your feature request related to a problem? Please describe.

In our Questionnaire we are collecting blood pressure values. We would like a typo checking mechanism to have the ability for the Questionnaire to give a warning prompt when the value of systolic is less than the value for diastolic.

Describe the solution you'd like

The use of the extension: https://www.hl7.org/fhir/extensions/5.1.0/StructureDefinition-targetConstraint.html looks like it would be a good fit. As it states a context of use:

Element ID Questionnaire
Element ID Questionnaire.item

Describe alternatives you've considered
N/A as yet- preference for targetConstraint

Additional context
The below is the example that I have tried in the playground renderer:

{
"resourceType": "Questionnaire",
"id": "example-questionnaire",
"status": "active",
"subjectType": ["Patient"],
"item": [
{
"linkId": "1.1",
"text": "Systolic Blood Pressure",
"type": "integer"
},
{
"linkId": "1.2",
"text": "Diastolic Blood Pressure",
"type": "integer"
}
],
"extension": [
{
"url": "http://hl7.org/fhir/StructureDefinition/targetConstraint",
"extension": [
{
"url": "key",
"valueId": "bp-constraint"
},
{
"url": "severity",
"valueCode": "warning"
},
{
"url": "expression",
"valueExpression": {
"language": "text/fhirpath",
"expression": "%resource.item.where(linkId='1.1').answer.value < %resource.item.where(linkId='1.2').answer.value"
}
},
{
"url": "human",
"valueString": "Systolic blood pressure should not be less than diastolic blood pressure."
}
]
}
]
}