anchor.diff:              let ``F'(x)`` = Map.find x F'
               Map.add x ``F'(x)`` F
   
  -        // Find the 'y' values related to 'x' and compute xRy
  -        // by recursively traversing them.
  +        // Find the 'y' values related to 'x' and compute xRy by recursively traversing them.
           let F, N, stack =
               match Map.tryFind x R with
               | None ->
  
positive.code:     and 'U : comparison> =
      Map<'T, Set<'U>>
  
  //
  [<RequireQualifiedAccess; CompilationRepresentation(CompilationRepresentationFlags.ModuleSuffix)>]
  module internal PartialFunction =
      //
      let empty : PartialFunction<'T, 'U> = Map.empty
  
      //
      let add source target (func : PartialFunction<'T,' U>) : PartialFunction<'T,' U> =
          let targets =
              match Map.tryFind source func with
  
negative.code:             F, N, stack
  
      /// <summary>The 'digraph' algorithm from DeRemer and Pennello's paper.</summary>
      /// <remarks>
      /// This algorithm quickly computes set relations by 'condensing' a relation graph's strongly-connected components (SCCs),
      /// then performing a bottom-up traversal of the resulting DAG.
      /// </remarks>
      /// <param name="X">The set on which the relation is defined.</param>
      /// <param name="R">A relation on <paramref name="X"/>.</param>
      /// <param name="F'">A function from <paramref name="X"/> to sets.</param>
      /// <returns><c>F</c>, a function from X to sets, such that <c>F x</c> satisfies equation 4.1 in DeRemer and Pennello's paper.</returns>
      let digraph (X : Set<'T>, R : Relation<'T>, F' : PartialFunction<'T, 'U>) =
          //
          let N =
  