diff --git a/src/PointSymmetric_Polytopic.jl b/src/PointSymmetric_Polytopic.jl
index 0eac5eb424..e1698a7e1d 100644
--- a/src/PointSymmetric_Polytopic.jl
+++ b/src/PointSymmetric_Polytopic.jl
@@ -26,3 +26,25 @@ subtypes(PointSymmetric_Polytopic)
 ```
 """
 abstract type PointSymmetric_Polytopic{N<:Real} <: PointSymmetric{N} end
+
+
+# --- common Polytopic functions (copy-pasted) ---
+
+
+"""
+    singleton_list(P::PointSymmetric_Polytopic{N})::Vector{Singleton{N}} where {N<:Real}
+
+Return the vertices of a polytopic as a list of singletons.
+
+### Input
+
+- `P` -- a polytopic set
+
+### Output
+
+List containing a singleton for each vertex.
+"""
+function singleton_list(P::PointSymmetric_Polytopic{N}
+                       )::Vector{Singleton{N}} where {N<:Real}
+    return [Singleton(vi) for vi in P.vertices_list]
+end
diff --git a/src/Polygonal.jl b/src/Polygonal.jl
index b1bf8aedaf..21ee34edc7 100644
--- a/src/Polygonal.jl
+++ b/src/Polygonal.jl
@@ -23,6 +23,9 @@ subtypes(Polygonal)
 abstract type Polygonal{N<:Real} <: Polytopic{N} end
 
 
+# --- LazySet interface functions ---
+
+
 """
     dim(P::Polygonal)::Int
 
diff --git a/src/Polytopic.jl b/src/Polytopic.jl
index a870fbd6fb..6c5068039f 100644
--- a/src/Polytopic.jl
+++ b/src/Polytopic.jl
@@ -1,5 +1,6 @@
 export Polytopic,
-       vertices_list
+       vertices_list,
+       singleton_list
 
 """
     Polytopic{N<:Real} <: LazySet
@@ -21,3 +22,24 @@ subtypes(Polytopic)
 ```
 """
 abstract type Polytopic{N<:Real} <: LazySet end
+
+
+# --- common Polytopic functions ---
+
+
+"""
+    singleton_list(P::Polytopic{N})::Vector{Singleton{N}} where {N<:Real}
+
+Return the vertices of a polytopic as a list of singletons.
+
+### Input
+
+- `P` -- a polytopic set
+
+### Output
+
+List containing a singleton for each vertex.
+"""
+function singleton_list(P::Polytopic{N})::Vector{Singleton{N}} where {N<:Real}
+    return [Singleton(vi) for vi in P.vertices_list]
+end
diff --git a/src/VPolygon.jl b/src/VPolygon.jl
index ca71cfb5fd..2aaa5ad88d 100644
--- a/src/VPolygon.jl
+++ b/src/VPolygon.jl
@@ -1,8 +1,6 @@
-import Base: <=, ∈
+import Base: ∈
 
-export VPolygon,
-       vertices_list,
-       singleton_list
+export VPolygon
 
 """
     VPolygon{N<:Real} <: Polygonal{N}
@@ -39,47 +37,9 @@ struct VPolygon{N<:Real} <: Polygonal{N}
     end
 end
 
-"""
-    σ(d::AbstractVector{<:Real}, P::VPolygon{N})::Vector{N} where {N<:Real}
-
-Return the support vector of a polygon in a given direction.
-
-### Input
-
-- `d` -- direction
-- `P` -- polygon in vertex representation
-
-### Output
-
-The support vector in the given direction.
-If the direction has norm zero, the first vertex is returned.
-
-### Algorithm
 
-This implementation performs a brute-force search, comparing the projection of
-each vector along the given direction.
-It runs in ``O(n)`` where ``n`` is the number of vertices.
+# --- Polygonal interface functions ---
 
-### Notes
-
-For arbitrary points without structure this is the best one can do.
-However, a more efficient approach can be used if the vertices of the polygon
-have been sorted in counter-clockwise fashion.
-In that case a binary search algorithm can be used that runs in ``O(\\log n)``.
-See issue [#40](https://github.com/JuliaReach/LazySets.jl/issues/40).
-"""
-function σ(d::AbstractVector{<:Real}, P::VPolygon{N})::Vector{N} where {N<:Real}
-    if isempty(P.vertices_list)
-        error("this polygon is empty")
-    end
-    i_max = 1
-    @inbounds for i in 2:length(P.vertices_list)
-        if dot(d, P.vertices_list[i] - P.vertices_list[i_max]) > zero(N)
-            i_max = i
-        end
-    end
-    return P.vertices_list[i_max]
-end
 
 """
     tovrep(P::VPolygon{N})::VPolygon{N} where {N<:Real}
@@ -115,6 +75,10 @@ function tohrep(P::VPolygon{N})::HPolygonal{N} where {N<:Real}
     error("this function is not implemented yet, see issue #5")
 end
 
+
+# --- Polytopic interface functions ---
+
+
 """
     vertices_list(P::VPolygon{N})::Vector{Vector{N}} where {N<:Real}
 
@@ -132,22 +96,50 @@ function vertices_list(P::VPolygon{N})::Vector{Vector{N}} where {N<:Real}
     return P.vertices_list
 end
 
+
+# --- LazySet interface functions ---
+
+
 """
-    singleton_list(P::VPolygon{N})::Vector{Singleton{N}} where {N<:Real}
+    σ(d::AbstractVector{<:Real}, P::VPolygon{N})::Vector{N} where {N<:Real}
 
-Return the vertices of a convex polygon in vertex representation as a list of
-singletons.
+Return the support vector of a polygon in a given direction.
 
 ### Input
 
-- `P` -- a polygon vertex representation
+- `d` -- direction
+- `P` -- polygon in vertex representation
 
 ### Output
 
-List containing a singleton for each vertex.
+The support vector in the given direction.
+If the direction has norm zero, the first vertex is returned.
+
+### Algorithm
+
+This implementation performs a brute-force search, comparing the projection of
+each vector along the given direction.
+It runs in ``O(n)`` where ``n`` is the number of vertices.
+
+### Notes
+
+For arbitrary points without structure this is the best one can do.
+However, a more efficient approach can be used if the vertices of the polygon
+have been sorted in counter-clockwise fashion.
+In that case a binary search algorithm can be used that runs in ``O(\\log n)``.
+See issue [#40](https://github.com/JuliaReach/LazySets.jl/issues/40).
 """
-function singleton_list(P::VPolygon{N})::Vector{Singleton{N}} where {N<:Real}
-    return [Singleton(vi) for vi in P.vertices_list]
+function σ(d::AbstractVector{<:Real}, P::VPolygon{N})::Vector{N} where {N<:Real}
+    if isempty(P.vertices_list)
+        error("this polygon is empty")
+    end
+    i_max = 1
+    @inbounds for i in 2:length(P.vertices_list)
+        if dot(d, P.vertices_list[i] - P.vertices_list[i_max]) > zero(N)
+            i_max = i
+        end
+    end
+    return P.vertices_list[i_max]
 end
 
 """