code: def startswith(self, event_property, value):
          
          c = self.copy()
          c.filters.append(filters.RE(event_property, "^{value}".format(
              value=value)))
          return c
docs: A starts-with filter chain.
  
          >>> request_time = EventExpression('request', 'elapsed_ms')
          >>> filtered = request_time.startswith('path', '/cube')
          >>> print(filtered)
          request(elapsed_ms).re(path, "^/cube")