Midrash su Levitico 14:57

16) (Vayikra 14:54) "This is the law, etc.": Whence is it derived that a Cohein who is expert in plague-spots (i.e., se'eth, sapachath, and bahereth), but not in nethakim; in nethakim, but not in karachoth; in karachoth, but not in shechin and michvah; in shechin and michvah, but not in garments; in garments, but not in houses — (Whence is it derived that) he should not examine plague-spots until he is expert in (all of) them and in their names? From (Vayikra 14:54-57) "This is the law for all plague-spots of leprosy and for a nethek and for the leprosy of a garment and of a house, and for a se'eth, and for a sapachath, and for a bahereth, to teach" (i.e., he should not teach others unless he is expert in all of them). (Vayikra 14:57) Rebbi says: "on the unclean day and on the clean day": We are hereby taught that he rules "unclean" in the daytime and he rules "clean" in the daytime. Chananiah b. Chachonai says: What is the intent of "to teach"? We are hereby taught that he does not examine plague-spots until his master teaches him.