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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 envcontext(patch, _env=None):
 """In this context, `os.environ` is modified according to `patch`. `patch` is an iterable of 2-tuples (key, value):
 `key`: string `value`: - string: `environ[key] == value` inside the context. - UNSET: `key not in environ` inside the context. - template: A template is a tuple of strings and Var which will be replaced with the previous environment """

    env = os.environ if _env is None else _env
    before = env.copy()

    for k, v in patch:
        if v is UNSET:
            env.pop(k, None)
        elif isinstance(v, tuple):
            env[k] = format_env(v, before)
        else:
            env[k] = v

    try:
        yield
    finally:
        env.clear()
        env.update(before)