Per discussion in https://github.com/apache/calcite/pull/1457, we should add TUMBLE to parser and rename it to "$TUMBLE".
"TUMBLE" as an operator name will be left for table-value function TUMBLE.
Support Table-valued Function TUMBLE in Planner
Grouped Window Function TUMBLE should return "$TUMBLE" as its operator name
Remove the first operand of RexCall from SqlWindowTableFunction
GitHub Pull Request #1481
GitHub Pull Request #1587