avoid method redefinition

Author: shashi

src/differentials.jl

@@ -188,13 +188,11 @@ for (modu, fun, arity) ∈ DiffRules.diffrules()
         else
             DiffRules.diffrule(modu, fun, ntuple(k->:(args[$k]), arity)...)[i]
         end
-        @eval derivative(::typeof($modu.$fun), args::NTuple{$arity,Any}, ::Val{$i}) = $expr
+        @eval derivative(::typeof($modu.$fun), args::NTuple{$arity,Any}, ::Val{$i}) = (x = $expr; !(x isa Expression || x isa Constant) ? Constant(x) : x)
     end
 end
 
 derivative(::typeof(+), args::NTuple{N,Any}, ::Val) where {N} = Constant(1)
-derivative(::typeof(-), args::NTuple{2,Any}, ::Val{1}) = Constant(1)
-derivative(::typeof(-), args::NTuple{2,Any}, ::Val{2}) = Constant(-1)
 derivative(::typeof(*), args::NTuple{N,Any}, ::Val{i}) where {N,i} = make_operation(*, deleteat!(collect(args), i))
 derivative(::typeof(one), args::Tuple{<:Any}, ::Val) = Constant(0)