ArtifactPredicate

One predicate: a path of object-key segments into the content, and an operator from the closed
PropertyOp set — reused, not copied (§3.2). There is no eq, regex, prefix or text-search
operator, so a resemblance-shaped question has no way to be written down.

{
    "op": {
        "op": "has_key"
    },
    "path": [
        "string"
    ]
}