code: def coordinates(value):
      
      if not value.strip():
          raise ValueError('Empty list of coordinates: %r' % value)
      points = []
      pointset = set()
      for i, line in enumerate(value.split(','), 1):
          pnt = point(line)
          if pnt[:2] in pointset:
              raise ValueError("Found overlapping site #%d, %s" % (i, line))
          pointset.add(pnt[:2])
          points.append(pnt)
      return points
docs: Convert a non-empty string into a list of lon-lat coordinates.
  
      >>> coordinates('')
      Traceback (most recent call last):
      ...
      ValueError: Empty list of coordinates: ''
      >>> coordinates('1.1 1.2')
      [(1.1, 1.2, 0.0)]
      >>> coordinates('1.1 1.2, 2.2 2.3')
      [(1.1, 1.2, 0.0), (2.2, 2.3, 0.0)]
      >>> coordinates('1.1 1.2 -0.4, 2.2 2.3 -0.5')
      [(1.1, 1.2, -0.4), (2.2, 2.3, -0.5)]
      >>> coordinates('0 0 0, 0 0 -1')
      Traceback (most recent call last):
      ...
      ValueError: Found overlapping site #2,  0 0 -1