A query DSL

[eikopf]

This is a longer-term goal, but it would be extremely useful for Gnomon to have a small DSL for querying against calendar documents (particularly for tool integrations and large calendars). There are a few design details to work out:

• Supposing we use names as unambiguous identifiers for nodes in a document, how do we talk about the document in relation to that name? Can we access parents, or children?
• What happens when a multi-step query fails in the middle?
• Will we permit transformations in the query (in the manner of jq selectors)?

I want to take my cues mostly from jq here, but I'd be interested to see what something like hledger or beancount does for handling programmatic tasks.