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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def BindVar(self, var_id, value):
 """Associates a value with given variable. This can be called multiple times to associate multiple values. Args: var_id: A variable id to bind the values to. value: A value to bind to the specified variable. Raises: KeyError: If given variable is not specified in the pattern. """

    if var_id not in self._vars:
      raise KeyError(var_id)

    self._var_bindings[var_id].append(value)