def convert_to_string(liste):
    """
    Function creates strings with '+' in place of ' '                                
    Input=list of string                      Output=string with + in place of ' '   
    """
    if not liste:
        return ''

    if len(liste) == 1:
        return liste[0]
    else:
        return liste[0] + '+' + convert_to_string(liste[1:])