ALGORITHM: find_common
    INPUT: list of files (file_list)
    DECLARE: hash of IDs (id_hash)
    CALL: get_data WITH references to file_list and id_hash
    CALL: get_common WITH reference to id_hash

FUNCTION: get_data
    INPUT: file_list and id_hash
    LOOP: file_list EACH file
        OPEN: file
        LOOP: read line EACH line
            remove line terminator
            split line INTO id and string
            LOOP: split string EACH substring
                increment id_hash-id-substring value
        CLOSE: file
    RETURN: void

FUNCTION: get_common
    INPUT: id_hash
    LOOP: id_hash EACH id
        LOOP: id_hash-id EACH substring
            SKIP: id_hash-id-substring value < 2
            PRINT: id : id_hash-id-substring key
    RETURN: void