def collapse(rng, start):
    rng.collapse(start)
base function to handle start/stop/restart watcher requests.
dataset/ETHPy150Open pyjs/pyjs/pyjswidgets/pyjamas/selection/Range.py/collapse--crossover with--dataset/ETHPy150Open circus-tent/circus/circus/commands/restart.py/execute_watcher_start_stop_restart