anchor.diff:                  Some <| flattenLV src]
                  |> List.choose id
   
  -    (ctx.MkAnd terms, new Set<string> (vars))
  +    (terms, new Set<string> (vars))
   
   /// Emits a Boolean fetch.
   let makeBoolFetch model dest src =
  
positive.code:     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 ] ))
  
  /// Emits Z3 corresponding to a prim.
  /// The result is a pair of the Z3 emission, and the set of names of
  
negative.code:                  // defined by the fetch mode.
                   makeFetchUpdate model srcE mode |> Some]
                  |> List.choose id
  
      // The variables whose post-states are bound are src, and, if
      // present, dest.
  