[
  {"op": "source", "role": "price", "kind": "F64"},
  {"op": "add", "type": "SMA", "name": "a",
   "args": {"window": "3"},
   "bind": {"length": {"I64": 2}}},
  {"op": "feed", "role": "price", "into": ["a.series"]},
  {"op": "expose", "from": "a.value", "as": "out"}
]