anchor.diff:      let destE = mkBoolLV ctx dest
       let srcE = mkBoolLV ctx src
   
  -    ( ctx.MkAnd [|makeRel model srcE destE
  -                  makeNoChange model srcE|],
  -      new Set<string> ( [ flattenLV dest
  -                          flattenLV src ] ))
  +    ( [makeRel model srcE destE
  +       makeNoChange model srcE],
  +      new Set<string> ( [flattenLV dest
  +                         flattenLV src] ))
   
   /// Emits Z3 corresponding to a prim.
   /// The result is a pair of the Z3 emission, and the set of names of
  
positive.code:                  // defined by the fetch mode.
                   makeFetchUpdate model srcE mode |> Some]
                  |> List.choose id
                  |> List.toArray
  
      // The variables whose post-states are bound are src, and, if
      // present, dest.
  
negative.code:     let failSem = mkAnd2 ctx (succDest, succTest)
      let failure = ctx.MkImplies (failCond, failSem)
  
      [success
       failure
       // This models set!after = set!before.
       makeNoChange model setE]
  
  /// Emits an arithmetic fetch.
  let makeArithFetch model dest src mode =
  