def add_variable_to_dataset(dataset, var_name):
    """
    Convenience function to add a variable to the dataset
    only if the variable doesn't already exist. This function
    is case sensitive.  "radius" and "Radius" are two different 
    variable names.
    returns the Variable object associated with the variable name
    """
   
    if var_name not in dataset.variable_names:
        return dataset.add_variable(var_name)
    return dataset.variable(var_name)