diff --git a/src/Tracker/lib.jl b/src/Tracker/lib.jl
index 3b2195f3..6003ebd0 100644
--- a/src/Tracker/lib.jl
+++ b/src/Tracker/lib.jl
@@ -1,12 +1,7 @@
 import Base: *
 
 toarray(xs::AbstractArray, ys::AbstractArray) = ys
-
-function toarray(xs::AbstractArray, y)
-  y′ = similar(xs, typeof(y), ())
-  y′[] = y
-  return y′
-end
+toarray(xs::AbstractArray, y) = similar(xs, typeof(y), ()) .= y
 
 Base.getindex(xs::TrackedArray, i...) =
   TrackedArray(Call(getindex, xs, i...), toarray(xs.x, xs.x[i...]))
@@ -21,13 +16,10 @@ Base.:-(xs::TrackedArray) = TrackedArray(Call(-, xs))
 
 # Reductions
 
+Base.sum(xs::TrackedArray, dim) = TrackedArray(Call(sum, xs, dim))
 Base.sum(xs::TrackedArray) = TrackedArray(Call(sum, xs), toarray(xs.x, sum(xs.x)))
 
-function back!(::typeof(sum), Δ, xs::TrackedArray)
-  Δ′ = similar(xs.x)
-  Δ′ .= Δ
-  back!(xs, Δ′)
-end
+back!(::typeof(sum), Δ, xs::TrackedArray, dim...) = back!(xs, similar(xs.x) .= Δ)
 
 # BLAS