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This section details on the original issue you should resolve

<issue_title>Support ImposeProfile in validation</issue_title>
<issue_description>The FHIR Extensions Package introduces the Impose Profile extension, which indicates additional profiles that a resource should be validated against: http://hl7.org/fhir/extensions/StructureDefinition-structuredefinition-imposeProfile.html

Instances can only be valid against this profile, if they also successfully validate against the profile referenced by this extension.
This is an instruction to validators to go and check the profile this extension refers to, and enforce that the instance being validated is valid against both source and target profiles. In general, the source profile won't restate the constraints in the target profile.

Please add support for this in the validator, that, when validating against a profile that has this extension, the instance is also validated against the additionally imposed profile.

Note: this is split out from the bigger SDK ticket on other 'multiple inheritance'-like solutions - #619
</issue_description>

Comments on the Issue (you are @copilot in this section)

@alexzautke Looking at the code of the Java validator, I can't currently see that this extension is being taken into account for instance validation. Only StructureDefinitions itself are validated to see if they are compatible. @wardweistra would you mind opening a Zulip discussion on this to see if Grahame has some reasons to not validate instances against the imposed profile?

Note that you can already solve this today using the SelectValidationProfiles element from the ValidationSettings of the validator.</comment_new>
<comment_new>@wardweistra
Ok, opened the discussion here: #implementers > imposeProfile validation</comment_new>
<comment_new>@wardweistra
@alexzautke Confirmed that it should#implementers > imposeProfile validation @ 💬 but may indeed not do that yet.</comment_new>
<comment_new>@wardweistra
So I guess we can start implementing this in parallel? 😇 </comment_new>
<comment_new>@alexzautke
Remember that there can be multiple imposeProfile extensions on the StructureDefinition!</comment_new>
<comment_new>@alexzautke
Add to the FS docs about validation that we support this extension</comment_new>