What suggested the form for the zero is, of course, purely a matter of conjecture. The dot, which the Hindus used to fill up lacunae in their manuscripts, much as we indicate a break in a sentence,[194] would have been a more natural symbol; and this is the one which the Hindus first used[195] and which most Arabs use to-day. There was also used for this purpose a cross, like our X, and this is occasionally found as a zero symbol.[196] In the Bakh[s.][=a]l[=i] manuscript above mentioned, the word _['s][=u]nya_, with the dot as its symbol, is used to denote the unknown quantity, as well as to denote zero. An analogous use of the {54} zero, for the unknown quantity in a proportion, appears in a Latin manuscript of some lectures by Gottfried Wolack in the University of Erfurt in 1467 and 1468.[197] The usage was noted even as early as the eighteenth century.[198] The small circle was possibly suggested by the spurred circle which was used for ten.[199] It has also been thought that the omicron used by Ptolemy in his _Almagest_, to mark accidental blanks in the sexagesimal system which he employed, may have influenced the Indian writers.[200] This symbol was used quite generally in Europe and Asia, and the Arabic astronomer Al-Batt[=a]n[=i][201] (died 929 A.D.) used a similar symbol in connection with the alphabetic system of numerals. The occasional use by Al-Batt[=a]n[=i] of the Arabic negative, _l[=a]_, to indicate the absence of minutes {55} (or seconds), is noted by Nallino.[202] Noteworthy is also the use of the [Circle] for unity in the ['S][=a]rad[=a] characters of the Kashmirian Atharva-Veda, the writing being at least 400 years old. Bh[=a]skara (c. 1150) used a small circle above a number to indicate subtraction, and in the Tartar writing a redundant word is removed by drawing an oval around it. It would be interesting to know whether our score mark [score mark], read "four in the hole," could trace its pedigree to the same sources. O'Creat[203] (c. 1130), in a letter to his teacher, Adelhard of Bath, uses [Greek: t] for zero, being an abbreviation for the word _teca_ which we shall see was one of the names used for zero, although it could quite as well be from [Greek: tziphra]. More rarely O'Creat uses [circle with bar], applying the name _cyfra_ to both forms. Frater Sigsboto[204] (c. 1150) uses the same symbol. Other peculiar forms are noted by Heiberg[205] as being in use among the Byzantine Greeks in the fifteenth century. It is evident from the text that some of these writers did not understand the import of the new system.[206]