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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 get_installed_version(vcs):
 """Get the installed version for this project. Args: vcs (easyci.vcs.base.Vcs) Returns: str - version number Raises: VersionNotInstalledError """

    version_path = _get_version_path(vcs)
    if not os.path.exists(version_path):
        raise VersionNotInstalledError
    with open(version_path, 'r') as f:
        return f.read().strip()