anchor.diff:                   let stack = us::stack in
                    norm cfg env stack t'
        
  -          | Tm_name x -> 
  -            rebuild cfg env stack t
  -           
             | Tm_fvar f -> 
               let should_delta = match cfg.delta_level with 
                   | NoDelta -> false
  
positive.code:  | UnivArgs of list<universe> * Range.range
   | MemoLazy of memo<(env * term)>
   | Match    of env * branches * Range.range
   | Abs      of env * binders * env * option<lcomp> * Range.range
   | App      of term * aqual * Range.range
   | Meta     of S.metadata * Range.range
  
  
negative.code:           | Tm_unknown
            | Tm_uvar _ 
            | Tm_constant _
            | Tm_name _
            | Tm_fvar( {fv_delta=Delta_constant} ) 
            | Tm_fvar( {fv_qual=Some Data_ctor } ) 
            | Tm_fvar( {fv_qual=Some (Record_ctor _)} ) -> //these last three are just constructors; no delta steps can apply
  