working on structural simplification

Author: vyudu

src/ModelingToolkit.jl

@@ -123,6 +123,7 @@ abstract type AbstractTimeIndependentSystem <: AbstractSystem end
 abstract type AbstractODESystem <: AbstractTimeDependentSystem end
 abstract type AbstractMultivariateSystem <: AbstractSystem end
 abstract type AbstractOptimizationSystem <: AbstractTimeIndependentSystem end
+abstract type AbstractDiscreteSystem <: AbstractTimeDependentSystem end
 
 function independent_variable end
 

src/systems/discrete_system/discrete_system.jl

@@ -17,7 +17,7 @@ eqs = [x(k+1) ~ σ*(y-x),
 @named de = DiscreteSystem(eqs)
 ```
 """
-struct DiscreteSystem <: AbstractTimeDependentSystem
+struct DiscreteSystem <: AbstractDiscreteSystem
     """
     A tag for the system. If two systems have the same tag, then they are
     structurally identical.

src/systems/discrete_system/implicit_discrete_system.jl

@@ -10,13 +10,13 @@ using ModelingToolkit: t_nounits as t
 @parameters σ=28.0 ρ=10.0 β=8/3 δt=0.1
 @variables x(t)=1.0 y(t)=0.0 z(t)=0.0
 k = ShiftIndex(t)
-eqs = [x ~ σ*(y(k-1)-x),
-       y ~ x*(ρ-z(k-1))-y,
-       z ~ x(k-1)*y - β*z]
+eqs = [x ~ σ*(y-x),
+       y ~ x*(ρ-z)-y,
+       z ~ x*y - β*z]
 @named ide = ImplicitDiscreteSystem(eqs,t,[x,y,z],[σ,ρ,β]; tspan = (0, 1000.0))
 ```
 """
-struct ImplicitDiscreteSystem <: AbstractTimeDependentSystem
+struct ImplicitDiscreteSystem <: AbstractDiscreteSystem
     """
     A tag for the system. If two systems have the same tag, then they are
     structurally identical.

src/systems/systemstructure.jl

@@ -430,9 +430,9 @@ function TearingState(sys; quick_cancel = false, check = true)
 
     ts = TearingState(sys, fullvars,
         SystemStructure(complete(var_to_diff), complete(eq_to_diff),
-            complete(graph), nothing, var_types, sys isa DiscreteSystem),
+                        complete(graph), nothing, var_types, sys isa AbstractDiscreteSystem),
         Any[])
-    if sys isa DiscreteSystem || sys isa ImplicitDiscreteSystem
+    if sys isa AbstractDiscreteSystem 
         ts = shift_discrete_system(ts)
     end
     return ts