According to the Radak, any root in which the first radical is the letter י is defective and quiescent (see Rule 2). And he considers the root יקף (to round) to be defective. And Rabbi Elia Levita disagrees with him and writes that its root is נקף; he holds that it would only be defective if the second radical were the letter צ. And moreover Rava and Rav Ashi disagree about this in Babylonian Talmud Makkot 20b with respect to the prohibition of rounding one’s temples⁶⁴. The Radak holds the opinion of Rava, while Rabbi Elia Levita holds the opinion of Rav Ashi.

footnotes: ⁶⁴ Leviticus 19:27.