code: def var(name, index=None):
      
      tname = type(name)
      if tname is str:
          names = (name, )
      elif tname is tuple:
          names = name
      else:
          fstr = "expected name to be a str or tuple, got {0.__name__}"
          raise TypeError(fstr.format(tname))
  
      if not names:
          raise ValueError("expected at least one name")
  
      for name in names:
          tname = type(name)
          if tname is not str:
              fstr = "expected name to be a str, got {0.__name__}"
              raise TypeError(fstr.format(tname))
  
      if index is None:
          indices = tuple()
      else:
          tindex = type(index)
          if tindex is int:
              indices = (index, )
          elif tindex is tuple:
              indices = index
          else:
              fstr = "expected index to be an int or tuple, got {0.__name__}"
              raise TypeError(fstr.format(tindex))
  
      for index in indices:
          tindex = type(index)
          if tindex is not int:
              fstr = "expected index to be an int, got {0.__name__}"
              raise TypeError(fstr.format(tindex))
          if index < 0:
              fstr = "expected index to be >= 0, got {}"
              raise ValueError(fstr.format(index))
  
      try:
          v = VARIABLES[(names, indices)]
      except KeyError:
          v = Variable(names, indices)
          VARIABLES[(names, indices)] = v
      return v
docs: Return a unique Variable instance.
  
      .. note::
         Do **NOT** call this function directly.
         Instead, use one of the concrete implementations:
  
         * :func:`pyeda.boolalg.bdd.bddvar`
         * :func:`pyeda.boolalg.expr.exprvar`,
         * :func:`pyeda.boolalg.table.ttvar`.