fix: fix vars!

Author: AayushSabharwal

src/utils.jl

@@ -350,20 +350,20 @@ function vars!(vars, eq::Equation; op = Differential)
     (vars!(vars, eq.lhs; op = op); vars!(vars, eq.rhs; op = op); vars)
 end
 function vars!(vars, O; op = Differential)
-    if isvariable(O) && !(istree(O) && operation(O) === getindex)
+    if isvariable(O)
         return push!(vars, O)
     end
-
     !istree(O) && return vars
+
+    operation(O) isa op && return push!(vars, O)
+
     if operation(O) === (getindex)
         arr = first(arguments(O))
-        return vars!(vars, arr)
+        istree(arr) && operation(arr) isa op && return push!(vars, O)
+       isvariable(arr) && return push!(vars, O)
     end
 
-    operation(O) isa op && return push!(vars, O)
-
     isvariable(operation(O)) && push!(vars, O)
-
     for arg in arguments(O)
         vars!(vars, arg; op = op)
     end