[
  {"op": "source", "role": "price", "kind": "F64"},
  {"op": "add", "type": "Session", "name": "sess",
   "args": {"tz": "berlin", "open": "09:30"},
   "bind": {"period_minutes": {"I64": 15}}},
  {"op": "feed", "role": "price", "into": ["sess.trigger"]},
  {"op": "expose", "from": "sess.bars_since_open", "as": "bars"}
]