code: def distance(f1, f2):
      
      if f1.end < f2.start: return f2.start - f1.end
      if f2.end < f1.start: return f1.start - f2.end
      return 0
docs: \
      Distance between 2 features. The integer result is always positive or zero.
      If the features overlap or touch, it is zero.
      >>> from intersecter import Feature, distance
      >>> distance(Feature(1, 2), Feature(12, 13))
      10
      >>> distance(Feature(1, 2), Feature(2, 3))
      0
      >>> distance(Feature(1, 100), Feature(20, 30))
      0