added starting functorutils docs

docs/make.jl

@@ -40,7 +40,7 @@ makedocs(
     "Decompositions"=>"pages/decompositions.md",
     "Deciding Sheaves"=>"pages/decidingsheaves.md",
 #    "Junction Trees"=>"pages/junction_trees.md",
-#    "Functor Utils"=>"pages/functor_utils.md",
+    "Functor Utils"=>"pages/functor_utils.md",
 #    "Nested UWDs"=>"pages/nested_uwds.md",
     "Library Reference"=>"api.md",
   ]

docs/src/pages/functor_utils.md

@@ -1 +1,21 @@
 # [Functor Utils] (@id FunctorUtils)
+
+Functor Utils only includes 4 functions and builds off of Decompositions.
+
+We first define the forgetful functors vs.
+
+```julia
+function vs(X::Graph) FinSet(length(vertices(X))) end
+function vs(f::ACSetTransformation) components(f)[1] end
+```
+
+We also define the functor skeleton taking set to the skeleton of the set.
+
+```julia
+function skeleton(s::FinSet) FinSet(length(s)) end
+function skeleton(f::FinFunction)
+  (dd, cc) = (dom(f), codom(f))
+  ℓ = isempty(dd) ? Int[] : [findfirst(item -> item == f(x), collect(cc)) for x ∈ collect(dd)]
+  FinFunction(ℓ, skeleton(dd), skeleton(cc))
+end
+```