documentation of org.apache.calcite.rel.core.Window.RexWinAggCall 😄

/**
   * A call to a windowed aggregate function.
   *
   * <p>Belongs to a {@link Window.Group}.
   *
   * <p>It's a bastard son of a {@link org.apache.calcite.rex.RexCall}; similar
   * enough that it gets visited by a {@link org.apache.calcite.rex.RexVisitor},
   * but it also has some extra data members.
   */