Cause
A custom constraint expected to resolve facets (@facet = "true") must supply code to implement, at minimum, a finish-facet function. Optionally, it may supply a start-facet function to support concurrent facet resolution. This error indicates that the finish-facet function has not been supplied, or that the options node configuring the custom constraint is incorrect.
Response
If the constraint should not resolve facets, set @facet ="false". If it should resolve facets, run search:check-options on the options node to ensure the configuration is valid.
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