User Story:

As a Metaschema developer, in order to share variable declarations for reuse in descendant child constraints without duplicate the same variable in every scope, I would like the Metaschema core syntax model to permit binding variable values with let but without needing to define a sibling allowed-values/expect/index/is-unique/etc. constraint beside it.

For an example as to why see PR review, specifically discussion in #542 (comment).

Goals:

• Adjust Metaschema module in schema/metaschema/metaschema-module-metaschema.xml to allow let bindings without a sibling constraint.
• Adjust Metaschema module in schema/xml/metaschema.xsd to allow let bindings without a sibling constraint.

Dependencies:

{Describe any previous issues or related work that must be completed to start or complete this issue.}

Acceptance Criteria

• All website and readme documentation affected by the changes in this issue have been updated. Changes to the website can be made in the docs/content directory of your branch.
• A Pull Request (PR) is submitted that fully addresses the goals of this User Story. This issue is referenced in the PR.
• The CI-CD build process runs without any reported errors on the PR. This can be confirmed by reviewing that all checks have passed in the PR.

{The items above are general acceptance criteria for all User Stories. Please describe anything else that must be completed for this issue to be considered resolved.}