anchor.diff:      and 'U : comparison> =
       Map<'T, Set<'U>>
   
  -//
  +/// Functions for working with PartialFunction instances.
   [<RequireQualifiedAccess; CompilationRepresentation(CompilationRepresentationFlags.ModuleSuffix)>]
   module internal PartialFunction =
  -    //
  +    /// Returns an empty partial function.
       let empty : PartialFunction<'T, 'U> = Map.empty
   
  -    //
  +    /// Returns a new partial function with the binding added to the given partial function.
       let add source target (func : PartialFunction<'T,' U>) : PartialFunction<'T,' U> =
           let targets =
               match Map.tryFind source func with
  
positive.code: /// A binary relation over a set of elements.
  // For each 'x', contains the 'y' values such that xRy (given a relation 'R').
  type internal Relation<'T when 'T : comparison> =
      Map<'T, Set<'T>>
  
  /// The traversal status of an element.
  type private TraversalStatus =
      /// The element has not yet been traversed.
      | NotStarted
      /// Traversal of the element is in-progress.
      | InProgress of int // depth
      /// Traversal of the element has been completed.
      | Completed
  
  //
  [<RequireQualifiedAccess; CompilationRepresentation(CompilationRepresentationFlags.ModuleSuffix)>]
  module internal Relation =
      //
      let empty : Relation<'T> = Map.empty
  
      //
      let add source target (relation : Relation<'T>) : Relation<'T> =
          let targets =
              match Map.tryFind source relation 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 =
  