Classifies a key without looking at its value, which is what splitting a node into its retrieval and constraint
halves takes; isCriterion holds the whole entry to its operator's contract instead.
True if value is a string composed of a Criteria operator prefix (<, >, <=, >=,
~, ?, !, +, ^) followed by a valid Expression, or exactly "@" or "#"; false otherwise
Checks if a value is a Criteria constraint key.
Classifies a key without looking at its value, which is what splitting a node into its retrieval and constraint halves takes; isCriterion holds the whole entry to its operator's contract instead.