def field_to_attr(field_name):
    """Convert a field name to an attribute name
    
    Make the field all lowercase and replace ' ' with '_'
    (replace space with underscore)
    """
    result = field_name.lower()
    if result[0:1].isdigit():
        result = "n_" + result
    result = result.replace(' ', '_')
    result = result.replace('/', '_')
    result = result.replace('-', '_')
    result = result.replace('.', '_')
    result = result.replace('+', '_plus')
    return result