    >>> userNameToFileName("t_h") == "t_h"
            True
            >>> userNameToFileName("F_F_I") == "F__F__I_"
            True
            >>> userNameToFileName("f_f_i") == "f_f_i"
            True
            >>> userNameToFileName("Aacute_V.swash") == "A_acute_V_.swash"
            True
            >>> userNameToFileName(".notdef") == "_notdef"
            True
            >>> userNameToFileName("con") == "_con"
            True
            >>> userNameToFileName("CON") == "C_O_N_"
            True
            >>> userNameToFileName("con.alt") == "_con.alt"
            True
            >>> userNameToFileName("alt.con") == "alt._con"
            True
    z(The value for userName must be a string.