Proposed Syntax

{ $_internalSchemaCond:
    [
        <conditionMatchExpression>,
        <thenBranchMatchExpression>,
       <elseBranchMatchExpression>
    ]
}

Proposed Semantics

A document is matched against the condition MatchExpression. If it matches, it is then matched against the thenBranch expression; otherwise, it is matched against the elseBranch expression. The document matches the entire $_internalSchemaCond expression based on whether or not it matches the thenBranch or elseBranch.

$_internalSchemaCond can be used to constrain a subobject if it is wrapped in an $_internalSchemaObjectMatch.