def get_node_file_output_str(nset, varstr, frequency=99999999):
    """ Get the string to add to the input file.
    
    :param nset: Name of node set from which output should be output
    :type nset: str
    
    :param varstr: comma separated list of variables to output
    :type varstr: str
    
    :param frequency: How often to write output (increments)
    :type frequency: int
    
    :returns: The string to add
    :rtype: str
    
    """
    
    output_str = ('*NODE FILE, NSET=' + nset + 
                  ', FREQUENCY=%0.0f \n'
                  + varstr) % (frequency)
    return output_str