i am looking for a train that leaves on tuesday to cambridge , please . i want to depart from stevenage and would like to arrive in cambridge by 16:30 .
kindly narrow down your specification -s so that i can find you a train that suits you best , like time of arrive and departure
{'service': 'train', 'act': 'request', 'slot_name': 'leave', 'value': '?'}
what time would you like to leave at ?