Added back ewf (reversing 1547dd6)
Added back ewf (reversing 1547dd6)

Substitution lemma was a complete mess without ewf

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