diff --git a/REQUIRE b/REQUIRE
index 8bb92ddb..95fda02c 100644
--- a/REQUIRE
+++ b/REQUIRE
@@ -1,5 +1,4 @@
 julia 0.6.0
-DataFlow 0.2.1
 Juno
 MacroTools 0.3.3
 NNlib
diff --git a/src/tracker/Tracker.jl b/src/tracker/Tracker.jl
index 4a58df29..d5f7dcfb 100644
--- a/src/tracker/Tracker.jl
+++ b/src/tracker/Tracker.jl
@@ -1,7 +1,7 @@
 module Tracker
 
 using MacroTools
-using MacroTools: @q
+using MacroTools: @q, @forward
 
 import Base: ==
 
@@ -71,6 +71,7 @@ function update!(x, Δ)
   return x
 end
 
+include("idset.jl")
 include("back.jl")
 include("scalar.jl")
 include("array.jl")
diff --git a/src/tracker/back.jl b/src/tracker/back.jl
index 3d769778..62cae1d0 100644
--- a/src/tracker/back.jl
+++ b/src/tracker/back.jl
@@ -52,6 +52,8 @@ back(x::Void, Δ) = error("Can't backpropagate through `nothing`")
 
 # TODO: if an error occurs in `back` the refcounts will be broken
 # and `back` will silently fail to update.
+# Refcounts are also probably not safe in some situations (e.g. back called
+# from within a backpropagator)
 
 function back!(x::Tracked, Δ)
   scan(x)
@@ -59,3 +61,88 @@ function back!(x::Tracked, Δ)
 end
 
 back!(x, Δ) = back!(tracker(x), Δ)
+
+# Out-of-place gradients
+
+struct Params
+  params::IdSet
+  Params(xs) = new(IdSet(xs))
+end
+
+@forward Params.params Base.start, Base.next, Base.done
+
+struct Grads
+  grads::ObjectIdDict
+end
+
+Grads() = Grads(ObjectIdDict())
+
+Base.getindex(g::Grads, x::Tracked) = g.grads[x]
+function Base.getindex(g::Grads, x)
+  istracked(x) || error("Object not tracked: $x")
+  g[tracker(x)]
+end
+
+@forward Grads.grads Base.setindex!, Base.haskey
+
+accum!(g::Grads, x, Δ) = g[x] = haskey(g, x) ? g[x] + Δ : Δ
+
+function back_(g::Grads, c::Call, Δ)
+  Δs = c.func(Δ)
+  (Δs isa Tuple && length(Δs) >= length(c.args)) ||
+    error("Gradient is not a tuple of length $(length(c.args))")
+  foreach((x, Δ) -> istracked(x) && back(g, x, Δ), c.args, Δs)
+end
+
+back_(g::Grads, ::Call{Void}, Δ) = nothing
+
+function back(g::Grads, x::Tracked, Δ)
+  x.isleaf && (accum!(g, x, Δ); return)
+  ref = x.ref -= 1
+  if ref > 0 || haskey(g, x)
+    accum!(g, x, Δ)
+    ref == 0 && back_(g, x.f, g[x])
+  else
+    ref == 0 && back_(g, x.f, Δ)
+  end
+  return
+end
+
+back(g::Grads, x, Δ) = back(g, tracker(x), Δ)
+back(g::Grads, x::Void, Δ) = error("Can't backpropagate through `nothing`")
+
+function forward(f, ps::Params)
+  y = f()
+  y, function (Δ)
+    g = Grads()
+    if istracked(y)
+      scan(y)
+      back(g, y, Δ)
+    end
+    for p in ps
+      haskey(g, tracker(p)) ||
+        (g[tracker(p)] = init_grad(data(p)))
+    end
+    return g
+  end
+end
+
+function forward(f, args...)
+  args = param.(args)
+  y, back = forward(() -> f(args...), Params(args))
+  y, Δ -> getindex.(back(Δ), args)
+end
+
+function losscheck(x)
+  x isa Real || error("Function output is not scalar")
+  isinf(x) && error("Loss is infinite")
+  isnan(x) && error("Loss is NaN")
+end
+
+function gradient(f, args...)
+  y, back = forward(f, args...)
+  losscheck(y)
+  return back(1)
+end
+
+derivative(f, x) = gradient(f, x)[1]
diff --git a/src/tracker/idset.jl b/src/tracker/idset.jl
new file mode 100644
index 00000000..68d1eea1
--- /dev/null
+++ b/src/tracker/idset.jl
@@ -0,0 +1,25 @@
+struct IdSet{T} <: AbstractSet{T}
+  dict::ObjectIdDict
+  IdSet{T}() where T = new(ObjectIdDict())
+end
+
+Base.eltype{T}(::IdSet{T}) = T
+
+IdSet() = IdSet{Any}()
+
+Base.push!{T}(s::IdSet{T}, x::T) = (s.dict[x] = nothing; s)
+Base.delete!{T}(s::IdSet{T}, x::T) = (delete!(s.dict, x); s)
+Base.in(x, s::IdSet) = haskey(s.dict, x)
+
+(::Type{IdSet{T}}){T}(xs) = push!(IdSet{T}(), xs...)
+
+IdSet(xs) = IdSet{eltype(xs)}(xs)
+
+Base.collect(s::IdSet) = Base.collect(keys(s.dict))
+Base.similar(s::IdSet, T::Type) = IdSet{T}()
+
+@forward IdSet.dict Base.length
+
+Base.start(s::IdSet) = start(keys(s.dict))
+Base.next(s::IdSet, st) = next(keys(s.dict), st)
+Base.done(s::IdSet, st) = done(keys(s.dict), st)
diff --git a/src/tracker/numeric.jl b/src/tracker/numeric.jl
index 755e1f7d..e0028b7c 100644
--- a/src/tracker/numeric.jl
+++ b/src/tracker/numeric.jl
@@ -1,9 +1,3 @@
-function gradient(f, xs...)
-  xs = param.(xs)
-  back!(f(xs...))
-  grad.(xs)
-end
-
 function ngradient(f, xs::AbstractArray...)
   grads = zeros.(xs)
   for (x, Δ) in zip(xs, grads), i in 1:length(x)
diff --git a/src/treelike.jl b/src/treelike.jl
index fbe9fcad..13e562e6 100644
--- a/src/treelike.jl
+++ b/src/treelike.jl
@@ -1,4 +1,5 @@
 import Adapt: adapt
+import .Tracker: IdSet
 
 children(x) = ()
 mapchildren(f, x) = x
@@ -20,9 +21,7 @@ function mapleaves(f, x; cache = ObjectIdDict())
   cache[x] = isleaf(x) ? f(x) : mapchildren(x -> mapleaves(f, x, cache = cache), x)
 end
 
-using DataFlow: OSet
-
-function prefor(f, x; seen = OSet())
+function prefor(f, x; seen = IdSet())
   x ∈ seen && return
   f(x)
   foreach(x -> prefor(f, x, seen = seen), children(x))