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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 add_variable(var, restore=True):
 """Adds a variable to the MODEL_VARIABLES collection. Optionally it will add the variable to the VARIABLES_TO_RESTORE collection. Args: var: a variable. restore: whether the variable should be added to the VARIABLES_TO_RESTORE collection. """

  collections = [MODEL_VARIABLES]
  if restore:
    collections.append(VARIABLES_TO_RESTORE)
  for collection in collections:
    if var not in tf.get_collection(collection):
      tf.add_to_collection(collection, var)