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12. perpendicular(): Return the opposite orientation

In this application, we often use 0 to denote a horizontal slot and 1 to denote a vertical slot. This utility routine inverts the sense of such a value. Assuming the value is either 0 or 1, subtracting the value from 1 will give the opposite value.

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# - - -   p e r p e n d i c u l a r

def perpendicular ( isV ):
    '''Invert an 'isV' flag.

      [ if isV is 0 ->
          return 1
        if isV is 1 ->
          return 0 ]
    '''
    return 1-isV