Tosefta על פאה 6:4

An ear of grain that is among the standing grain, behold, it belongs to the homeowner. [An ear of grain] that is among the harvested [grain], behold, it belongs to the poor. [If] half is among the standing grain, and half is among the harvested [grain], he takes it and throws it behind him, because [in a case of] doubt [as to what is considered] gleanings, [the doubt is resolved in favor of its status being] gleanings (see also Peah 6:4 re: forgotten sheaves).

What is [the case mentioned in the Mishna titled as] “a sheaf that is in front of it proves [that it is not considered to be Shikcha”]? Someone who had 10 rows [of sheaves with] 10 sheaves in each row, and he stooked one of [the rows in the direction] from North to South, and [then] forgot [the last sheaf in that row.] It is not [considered to be] Shikcha (forgotten sheaf), because [that sheaf] can be counted [as a part of the last row that goes] from East to West.