Added excluded middle lemma
Added excluded middle lemma

YaccConstructor/FStar,msprotz/FStar,aa755/FStar,MallabaevAzamat/FStar,A-Manning/FStar,msprotz/FStar,hoheinzollern/FStar,FStarLang/FStar,FStarLang/FStar,YaccConstructor/FStar,MallabaevAzamat/FStar,A-Manning/FStar,FStarLang/FStar,hoheinzollern/FStar,aa755/FStar,MallabaevAzamat/FStar,aa755/FStar,aa755/FStar,MallabaevAzamat/FStar,msprotz/FStar,YaccConstructor/FStar,hoheinzollern/FStar,A-Manning/FStar,hoheinzollern/FStar,YaccConstructor/FStar,msprotz/FStar