Address code review comments

Author: YingboMa

src/systems/diffeqs/abstractodesystem.jl

@@ -418,3 +418,5 @@ end
 function SteadyStateProblemExpr(sys::AbstractODESystem, args...; kwargs...)
     SteadyStateProblemExpr{true}(sys, args...; kwargs...)
 end
+
+isdiffeq(eq) = eq.lhs isa Term && operation(eq.lhs) isa Differential

src/systems/reduction.jl

@@ -49,7 +49,7 @@ function get_α_x(αx)
     end
 end
 
-function alias_elimination(sys)
+function alias_elimination(sys::ODESystem)
     eqs = vcat(equations(sys), observed(sys))
 
     subs = Pair[]