problem-set/topology/point.set/lemma22.desc created : 08/08/88 revised : 08/17/89 Natural Language Description: Lemma : boundary(xa) is empty iff xa is both open and closed. Versions : lemma22 : Standard axiom set, using typed predicates; linked UR should work. created : from Wick & McCune, 1989. verified for ITP : untested. translated for OTTER by : caw. verified for OTTER : untested.