feat: add is_transparent_operator trait

Author: AayushSabharwal

src/discretedomain.jl

@@ -1,5 +1,15 @@
 using Symbolics: Operator, Num, Term, value, recursive_hasoperator
 
+"""
+    $(TYPEDSIGNATURES)
+
+Trait to be implemented for operators which determines whether application of the operator
+generates a semantically different variable or not. For example, `Differential` and `Shift`
+are not transparent but `Sample` and `Hold` are. Defaults to `false` if not implemented.
+"""
+is_transparent_operator(x) = is_transparent_operator(typeof(x))
+is_transparent_operator(::Type) = false
+
 """
     function SampleTime()
 
@@ -128,6 +138,8 @@ struct Sample <: Operator
     Sample(clock::Union{TimeDomain, InferredTimeDomain} = InferredDiscrete()) = new(clock)
 end
 
+is_transparent_operator(::Type{Sample}) = true
+
 function Sample(arg::Real)
     arg = unwrap(arg)
     if symbolic_type(arg) == NotSymbolic()
@@ -179,6 +191,9 @@ cont_x = Hold()(disc_x)
 """
 struct Hold <: Operator
 end
+
+is_transparent_operator(::Type{Hold}) = true
+
 (D::Hold)(x) = Term{symtype(x)}(D, Any[x])
 (D::Hold)(x::Num) = Num(D(value(x)))
 SymbolicUtils.promote_symtype(::Hold, x) = x

src/systems/systemstructure.jl

@@ -363,7 +363,7 @@ function TearingState(sys; quick_cancel = false, check = true, sort_eqs = true)
             isdelay(v, iv) && continue
 
             if !symbolic_contains(v, dvs)
-                isvalid = iscall(v) && operation(v) isa Union{Shift, Sample, Hold}
+                isvalid = iscall(v) && (operation(v) isa Shift || is_transparent_operator(operation(v)))
                 v′ = v
                 while !isvalid && iscall(v′) && operation(v′) isa Union{Differential, Shift}
                     v′ = arguments(v′)[1]