LET sstring$ = "[[1], 2, [[3,4], 5], [[[]]], [[[6]]], 7, 8 []]"
FOR sicount = 1 TO LEN(sstring$)
    IF pos("[] ,",(sstring$)[sicount:sicount+1-1]) = 0 THEN
       LET sflatter$ = sflatter$ & scomma$ & (sstring$)[sicount:sicount+1-1]
       LET scomma$ = ", "
    END IF
NEXT sicount
PRINT "["; sflatter$; "]"
END