Syntax trees as JSON

Standard Datalog has only flat, atomic values. Datamog adds one more type, value: the union of every JSON shape (null, numbers, strings, booleans, arrays and objects), enough to hold a whole syntax tree.

Make each node an object tagged with its type, put its children in an args array, and let a variable carry its name. So p & q becomes:

{"type": "and",
 "args": [{"type": "var", "name": "p"},
          {"type": "var", "name": "q"}]}