* Fixed a type signature that was incorrect.
* Also, replaced `data(a)` with `a.data`. Don't know if the syntax has changed (recently). This may also need to be corrected in line 121.
MWE:
```julia
using Flux
using Flux.Tracker
using Flux.Tracker: forward, TrackedReal, track, @grad
minus(a, b) = a - b
minus(a::TrackedReal, b::TrackedReal) = Tracker.track(minus, a, b)
@grad function minus(a, b)
return minus(a.data, b.data), Δ -> (Δ, -Δ)
end
a, b = param(2), param(4)
c = minus(a, b) # -2.0 (tracked)
Tracker.back!(c)
Tracker.grad(a) # 1.00
Tracker.grad(b) # -1.00
```
