add trait and dispatch for collect_vars!

src/systems/diffeqs/odesystem.jl

@@ -319,8 +319,7 @@ function ODESystem(eqs, iv; kwargs...)
     compressed_eqs = Equation[] # equations that need to be expanded later, like `connect(a, b)`
     for eq in eqs
         eq.lhs isa Union{Symbolic, Number} || (push!(compressed_eqs, eq); continue)
-        collect_vars!(allunknowns, ps, eq.lhs, iv)
-        collect_vars!(allunknowns, ps, eq.rhs, iv)
+        collect_vars!(allunknowns, ps, eq, iv)
         if isdiffeq(eq)
             diffvar, _ = var_from_nested_derivative(eq.lhs)
             if check_scope_depth(getmetadata(diffvar, SymScope, LocalScope()), 0)
@@ -337,11 +336,9 @@ function ODESystem(eqs, iv; kwargs...)
     end
     for eq in get(kwargs, :parameter_dependencies, Equation[])
         if eq isa Pair
-            collect_vars!(allunknowns, ps, eq[1], iv)
-            collect_vars!(allunknowns, ps, eq[2], iv)
+            collect_vars!(allunknowns, ps, eq, iv)
         else
-            collect_vars!(allunknowns, ps, eq.lhs, iv)
-            collect_vars!(allunknowns, ps, eq.rhs, iv)
+            collect_vars!(allunknowns, ps, eq, iv)
         end
     end
     for ssys in get(kwargs, :systems, ODESystem[])

src/systems/discrete_system/discrete_system.jl

@@ -175,8 +175,7 @@ function DiscreteSystem(eqs, iv; kwargs...)
     ps = OrderedSet()
     iv = value(iv)
     for eq in eqs
-        collect_vars!(allunknowns, ps, eq.lhs, iv; op = Shift)
-        collect_vars!(allunknowns, ps, eq.rhs, iv; op = Shift)
+        collect_vars!(allunknowns, ps, eq, iv; op = Shift)
         if iscall(eq.lhs) && operation(eq.lhs) isa Shift
             isequal(iv, operation(eq.lhs).t) ||
                 throw(ArgumentError("A DiscreteSystem can only have one independent variable."))
@@ -187,11 +186,9 @@ function DiscreteSystem(eqs, iv; kwargs...)
     end
     for eq in get(kwargs, :parameter_dependencies, Equation[])
         if eq isa Pair
-            collect_vars!(allunknowns, ps, eq[1], iv)
-            collect_vars!(allunknowns, ps, eq[2], iv)
+            collect_vars!(allunknowns, ps, eq, iv)
         else
-            collect_vars!(allunknowns, ps, eq.lhs, iv)
-            collect_vars!(allunknowns, ps, eq.rhs, iv)
+            collect_vars!(allunknowns, ps, eq, iv)
         end
     end
     new_ps = OrderedSet()

src/systems/nonlinear/nonlinearsystem.jl

@@ -166,16 +166,13 @@ function NonlinearSystem(eqs; kwargs...)
     allunknowns = OrderedSet()
     ps = OrderedSet()
     for eq in eqs
-        collect_vars!(allunknowns, ps, eq.lhs, nothing)
-        collect_vars!(allunknowns, ps, eq.rhs, nothing)
+        collect_vars!(allunknowns, ps, eq, nothing)
     end
     for eq in get(kwargs, :parameter_dependencies, Equation[])
         if eq isa Pair
-            collect_vars!(allunknowns, ps, eq[1], nothing)
-            collect_vars!(allunknowns, ps, eq[2], nothing)
+            collect_vars!(allunknowns, ps, eq, nothing)
         else
-            collect_vars!(allunknowns, ps, eq.lhs, nothing)
-            collect_vars!(allunknowns, ps, eq.rhs, nothing)
+            collect_vars!(allunknowns, ps, eq, nothing)
         end
     end
     new_ps = OrderedSet()

src/utils.jl

@@ -492,20 +492,19 @@ recursively searches through all subsystems of `sys`, increasing the depth if it
 function collect_scoped_vars!(unknowns, parameters, sys, iv; depth = 1, op = Differential)
     if has_eqs(sys)
         for eq in get_eqs(sys)
-            eq isa Equation || continue
-            eq.lhs isa Union{Symbolic, Number} || continue
-            collect_vars!(unknowns, parameters, eq.lhs, iv; depth, op)
-            collect_vars!(unknowns, parameters, eq.rhs, iv; depth, op)
+            eqtype_supports_collect_vars(eq) || continue
+            if eq isa Equation
+                eq.lhs isa Union{Symbolic, Number} || continue
+            end
+            collect_vars!(unknowns, parameters, eq, iv; depth, op)
         end
     end
     if has_parameter_dependencies(sys)
         for eq in get_parameter_dependencies(sys)
             if eq isa Pair
-                collect_vars!(unknowns, parameters, eq[1], iv; depth, op)
-                collect_vars!(unknowns, parameters, eq[2], iv; depth, op)
+                collect_vars!(unknowns, parameters, eq, iv; depth, op)
             else
-                collect_vars!(unknowns, parameters, eq.lhs, iv; depth, op)
-                collect_vars!(unknowns, parameters, eq.rhs, iv; depth, op)
+                collect_vars!(unknowns, parameters, eq, iv; depth, op)
             end
         end
     end
@@ -529,6 +528,29 @@ function collect_vars!(unknowns, parameters, expr, iv; depth = 0, op = Different
     return nothing
 end
 
+"""
+    $(TYPEDSIGNATURES)
+
+Indicate whether the given equation type (Equation, Pair, etc) supports `collect_vars!`. 
+Can be dispatched by higher-level libraries to indicate support.
+"""
+eqtype_supports_collect_vars(eq) = false
+eqtype_supports_collect_vars(eq::Equation) = true
+eqtype_supports_collect_vars(eq::Pair) = true
+
+function collect_vars!(unknowns, parameters, eq::Equation, iv;
+        depth = 0, op = Differential)
+    collect_vars!(unknowns, parameters, eq.lhs, iv; depth, op)
+    collect_vars!(unknowns, parameters, eq.rhs, iv; depth, op)
+    return nothing
+end
+
+function collect_vars!(unknowns, parameters, p::Pair, iv; depth = 0, op = Differential)
+    collect_vars!(unknowns, parameters, p[1], iv; depth, op)
+    collect_vars!(unknowns, parameters, p[2], iv; depth, op)
+    return nothing
+end
+
 function collect_var!(unknowns, parameters, var, iv; depth = 0)
     isequal(var, iv) && return nothing
     check_scope_depth(getmetadata(var, SymScope, LocalScope()), depth) || return nothing