def combine_usub_files(file_list):
    """ Combine the user subroutine files in file_list to one file. Any 
    `!DEC$ FREEFORM` in the files are commented and `!DEC$ FREEFORM` is
    added to the top of the file. Includes can still be used in these 
    files, so only files containing Abaqus subroutines should be in this
    list.
    
    :param file_list: List of files to be combined
    :type file_list: List[ str ]
    
    :returns: Name of the combined user subroutine file
    :rtype: str
    
    """
    
    combined_file_name = 'usubs_combined.for'
    with open(combined_file_name, 'w') as fid:
        fid.write('!DEC$ FREEFORM\n')
        for file in file_list:
            with open(file, 'r') as sub:
                sub_str = sub.read()
            sub_str = sub_str.replace('!DEC$ FREEFORM', '!!DEC$ FREEFORM')
            fid.write('\n! Contents from ' + file + '\n')
            fid.write(sub_str)
            
    return combined_file_name