refactor: fix SII for Symbol fallbacks

src/systems/abstractsystem.jl

@@ -184,10 +184,13 @@ end
 
 #Treat the result as a vector of symbols always
 function SymbolicIndexingInterface.is_variable(sys::AbstractSystem, sym)
-    issym(sym) || return false
     if unwrap(sym) isa Int    # [x, 1] coerces 1 to a Num
         return unwrap(sym) in 1:length(unknown_states(sys))
     end
+    usym = unwrap(sym)
+    if !(istree(usym) && (operation(usym) == getindex || !isa(operation(usym), Function)))
+        return false
+    end
     return any(isequal(sym), unknown_states(sys)) || is_variable(sys, getname(sym))
 end
 
@@ -221,7 +224,10 @@ function SymbolicIndexingInterface.variable_symbols(sys::AbstractSystem)
 end
 
 function SymbolicIndexingInterface.is_parameter(sys::AbstractSystem, sym)
-    issym(sym) || return false
+    if unwrap(sym) isa Int
+        return unwrap(sym) in 1:length(parameters(sys))
+    end
+
     return any(isequal(sym), parameters(sys)) || is_parameter(sys, getname(sym))
 end