The reading in the digitized British Library manuscript differs from the published versions of the line which use a grammatically sanitized text. We have corrected the text provided us from the Clarke/Aufrecht edition accordingly. The form itself, however, may be an error in any case. If correct it must be taken as a nominalized passive participle from the verb gnn #2 "to construct a canopy" and refer to the frequent concern in the laws of purity to the difference between contamination from the air in an enclosed space and an open one. Nor can the reading כגנונא, i.e., "like a canopy", be excluded. We assume that is what the scribal correction intended.