lang/c++/jsonschemas/union_map_union

{
  "type": "record",
  "name": "r1",
  "fields" : [
    {"name": "id", "type": "string"},
    {"name": "val", "type": [{"type": "map", "values": {"name": "r3", "type": "record", "fields": [{"name": "name", "type": "string"}, {"name": "data", "type": "bytes"}, {"name": "rev", "type": ["string", "null"]}]}}, "null"]}
  ]
}