Allowing externals in unions

[stephencroberts]

Proposal: Externals should be allowed in a union if they are not extended or mixed with internals.

#%RAML 1.0
---
types:
  A:
    type: !include A.schema.json
  B:
    type: !include B.schema.json
title: externals with union
baseUri: https://example.com
/resource:
  post:
    body:
      application/json:
        type: A | B

The above currently receives the following error message:

EXTERNALS_MIX: It is not allowed to mix RAML types with externals

If I alter the above example to use RAML types instead of JSON Schemas, the union works as expected; however, I lose the benefits of extracting and using JSON schemas.

It's also important to note that I'm using raml2html to validate my RAML document.

Would the community be open to allowing this behavior?