diff --git a/src/diagrams/diagrams.jl b/src/diagrams/diagrams.jl
index a4d653c..2fc6ee7 100644
--- a/src/diagrams/diagrams.jl
+++ b/src/diagrams/diagrams.jl
@@ -65,17 +65,16 @@ struct FeynmanDiagram{N,E,U,T,M,FM} <: AbstractTreeLevelFeynmanDiagram where {N,
         @assert T == length(tauon_perm)
         N = E + U + T
 
-        fm = FlatMatrix(structure)
-        return new{N,E,U,T,M,typeof(fm)}(fm, NTuple{E,Int}(elec_perm), NTuple{U,Int}(muon_perm), NTuple{T,Int}(tauon_perm))
+        return new{N,E,U,T,M,FlatMatrix{Int64,2 * N - 2 + M,N}}(FlatMatrix(structure), NTuple{E,Int}(elec_perm), NTuple{U,Int}(muon_perm), NTuple{T,Int}(tauon_perm))
     end
 end
 
-function virtual_particles(diagram::FeynmanDiagram{N,E,U,T,M,FM})
+function virtual_particles(diagram::FeynmanDiagram{N,E,U,T,M,FM}) where {N,E,U,T,M,FM}
 
     return NTuple{N,Tuple{QEDbase.AbstractParticleType,BitArray,BitArray}}()
 end
 
-function vertices(::AbstractTreeLevelFeynmanDiagram)
+function vertices(diagram::FeynmanDiagram{N,E,U,T,M,FM}) where {N,E,U,T,M,FM}
 
     return NTuple{N,VertexType}()
 end
@@ -174,7 +173,8 @@ function Base.length(it::FeynmanDiagramIterator{E,U,T,M}) where {E,U,T,M}
     return factorial(M + 3 * N - 3, 2 * N - 1) * factorial(E) * factorial(U) * factorial(T)
 end
 
-function Base.iterate(iter::FeynmanDiagramIterator)
+function Base.iterate(iter::FeynmanDiagramIterator{E,U,T,M}) where {E,U,T,M}
+    N = E + U + T
     f = FeynmanDiagram(iter.photon_structure, iter.e_perms[iter.e_index], iter.u_perms[iter.u_index], iter.t_perms[iter.t_index], iter.e, iter.u, iter.t, iter.m)
     return (
         f,
@@ -182,7 +182,7 @@ function Base.iterate(iter::FeynmanDiagramIterator)
     )
 end
 
-function Base.iterate(iter::FeynmanDiagramIterator, ::Nothing)
+function Base.iterate(iter::FeynmanDiagramIterator{E,U,T,M}, ::Nothing) where {E,U,T,M}
     iter.t_index += 1
 
     if iter.t_index > length(iter.t_perms)
@@ -204,6 +204,7 @@ function Base.iterate(iter::FeynmanDiagramIterator, ::Nothing)
         (iter.photon_structure, _) = photon_iter_result
     end
 
+    N = E + U + T
     f = FeynmanDiagram(iter.photon_structure, iter.e_perms[iter.e_index], iter.u_perms[iter.u_index], iter.t_perms[iter.t_index], iter.e, iter.u, iter.t, iter.m)
     return (
         f,
diff --git a/src/generic_process_def.jl b/src/generic_process_def.jl
index 311b1cc..38bb99d 100644
--- a/src/generic_process_def.jl
+++ b/src/generic_process_def.jl
@@ -16,6 +16,17 @@ struct GenericQEDProcess{INT,OUTT} <: AbstractProcessDefinition where {INT<:Tupl
 
         return new{INT,OUTT}(in_particles, out_particles)
     end
+
+    """
+        GenericQEDProcess(in_ph::Int, out_ph::Int, in_el::Int, out_el::Int, in_po::Int, out_po::Int)
+
+    Convenience constructor from numbers of input/output photons, electrons and positrons.
+    """
+    function GenericQEDProcess(in_ph::Int, out_ph::Int, in_el::Int, out_el::Int, in_po::Int, out_po::Int)
+        in_p = ntuple(i -> i <= in_ph ? Photon() : i <= in_ph + in_el ? Electron() : Positron(), in_ph + in_el + in_po)
+        out_p = ntuple(i -> i <= out_ph ? Photon() : i <= out_ph + out_el ? Electron() : Positron(), out_ph + out_el + out_po)
+        return GenericQEDProcess(in_p, out_p)
+    end
 end
 
 QEDprocesses.incoming_particles(proc::GenericQEDProcess) = proc.incoming_particles