diff --git a/Project.toml b/Project.toml
index 0fbe2554..7379eaa2 100644
--- a/Project.toml
+++ b/Project.toml
@@ -27,7 +27,7 @@ ADNLPModels = "0.8"
 CTBase = "0.13"
 DocStringExtensions = "0.9"
 HSL = "0.4"
-JLD2 = "0.5"
+JLD2 = "0.4" # 0.5 has ugly warnings for functions
 JSON3 = "1"
 MadNLP = "0.8"
 NLPModels = "0.21"
diff --git a/docs/Project.toml b/docs/Project.toml
index 098fd971..7d603782 100644
--- a/docs/Project.toml
+++ b/docs/Project.toml
@@ -15,7 +15,7 @@ CTDirect = "0.12"
 Documenter = "1"
 DocumenterMermaid = "0.1"
 HSL = "0.4"
-JLD2 = "0.5"
+JLD2 = "0.4" # 0.5 has ugly warnings for functions
 JSON3 = "1"
 NLPModelsIpopt = "0.10"
 Plots = "1"
diff --git a/src/midpoint.jl b/src/midpoint.jl
index 0ce639d5..a6100c6a 100644
--- a/src/midpoint.jl
+++ b/src/midpoint.jl
@@ -5,11 +5,17 @@ with the convention u([t_i,t_i+1[) = U_i and u(tf) = U_N-1
 =#
 
 # +++ TODO: use args
-
+# NB. could be defined as a generic IRK
 struct Midpoint <: Discretization
+
     stage::Int
     additional_controls::Int
-    Midpoint() = new(1, 0)
+    butcher_a::Matrix{Float64}
+    butcher_b::Vector{Float64}
+    butcher_c::Vector{Float64}
+    info::String
+
+    Midpoint() = new(1, 0, hcat(0.5), [1], [0.5], "Implicit Midpoint aka Gauss-Legendre collocation for s=1, 2nd order, symplectic")
 end
 
 """
@@ -40,7 +46,7 @@ function get_variables_at_time_step(xu, docp::DOCP{Midpoint}, i)
     if i < N
         offset_u = offset
     else
-        offset_u = (nx * 2 + m) * (i - 1)
+        offset_u = (nx * (1 + docp.discretization.stage) + m) * (i - 1)
     end
     if m == 1
         ui = xu[offset_u + nx + 1]
@@ -66,7 +72,7 @@ function get_NLP_variables_at_time_step(xu, docp, i, disc::Midpoint)
     nx = docp.dim_NLP_x
     m = docp.dim_NLP_u
     N = docp.dim_NLP_steps
-    offset = (nx * 2 + m) * i
+    offset = (nx * (1 + docp.discretization.stage) + m) * i
 
     # state
     xi = xu[(offset + 1):(offset + nx)]
@@ -74,7 +80,7 @@ function get_NLP_variables_at_time_step(xu, docp, i, disc::Midpoint)
     if i < N
         offset_u = offset
     else
-        offset_u = (nx * 2 + m) * (i - 1)
+        offset_u = (nx * (1 + docp.discretization.stage) + m) * (i - 1)
     end
     ui = xu[(offset_u + nx + 1):(offset_u + nx + m)]
     # stage
@@ -92,7 +98,7 @@ function set_variables_at_time_step!(xu, x_init, u_init, docp, i, disc::Midpoint
     n = docp.dim_OCP_x
     m = docp.dim_NLP_u
     N = docp.dim_NLP_steps
-    offset = (nx * 2 + m) * i
+    offset = (nx * (1 + docp.discretization.stage) + m) * i
 
     # NB. only set the actual state variables from the OCP 
     # - skip the possible additional state for lagrange cost
@@ -155,9 +161,9 @@ $(TYPEDSIGNATURES)
 Set the constraints corresponding to the state equation
 """
 function setStateEquation!(docp::DOCP{Midpoint}, c, index::Int, args, v, i)
+    
     ocp = docp.ocp
-
-    # +++ later use butcher table in struct ?
+    disc = docp.discretization
 
     # variables
     ti = args.time
@@ -171,16 +177,21 @@ function setStateEquation!(docp::DOCP{Midpoint}, c, index::Int, args, v, i)
     hi = tip1 - ti
 
     # midpoint rule
-    @. c[index:(index + docp.dim_OCP_x - 1)] = xip1 - (xi + hi * ki[1:(docp.dim_OCP_x)])
+    h_sum_bk = hi * disc.butcher_b[1] * ki[1:docp.dim_NLP_x]
+    c[index:(index + docp.dim_OCP_x - 1)] .= xip1 .- (xi .+ h_sum_bk[1:docp.dim_OCP_x])
     # +++ just define extended dynamics !
     if docp.has_lagrange
-        c[index + docp.dim_OCP_x] = xlip1 - (xli + hi * ki[end])
+        c[index + docp.dim_OCP_x] = xlip1 - (xli + h_sum_bk[end])
     end
     index += docp.dim_NLP_x
 
     # stage equation at mid-step
-    t_s = 0.5 * (ti + tip1)
-    x_s = 0.5 * (xi + xip1)
+    t_s = ti + hi * disc.butcher_c[1]
+    if docp.dim_OCP_x == 1
+        x_s = xi + hi * disc.butcher_a[1][1] * ki[1] #FFS
+    else
+        x_s = xi .+ hi .* (disc.butcher_a[1][1] .* ki[1:docp.dim_OCP_x])
+    end
     c[index:(index + docp.dim_OCP_x - 1)] .= ki[1:(docp.dim_OCP_x)] .- ocp.dynamics(t_s, x_s, ui, v)
     # +++ just define extended dynamics !
     if docp.has_lagrange
diff --git a/src/trapeze.jl b/src/trapeze.jl
index 18878300..f2b5c8ef 100644
--- a/src/trapeze.jl
+++ b/src/trapeze.jl
@@ -3,11 +3,14 @@ Internal layout for NLP variables:
 [X_0,U_0, X_1,U_1, .., X_N,U_N, V]
 =#
 
+# NB. could be defined as a generic IRK
 struct Trapeze <: Discretization
+
     stage::Int
-    additional_controls::Int
-    # add control at tf
-    Trapeze() = new(0, 1)
+    additional_controls::Int  # add control at tf
+    info::String
+
+    Trapeze() = new(0, 1, "Implicit Trapeze aka Crank-Nicolson, 2nd order, A-stable")
 end
 
 """
diff --git a/test/Project.toml b/test/Project.toml
index ed0bdd43..ca19863c 100644
--- a/test/Project.toml
+++ b/test/Project.toml
@@ -13,7 +13,7 @@ Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 [compat]
 CTBase = "0.13"
 HSL = "0.4"
-JLD2 = "0.5"
+JLD2 = "0.4" # 0.5 has ugly warnings for functions
 JSON3 = "1"
 MadNLP = "0.8"
 NLPModelsIpopt = "0.10"