Minor tidy: use peel rather than [1] and drop (#483)

Author: oxinabox

src/rule_definition_tools.jl

@@ -294,8 +294,8 @@ function propagation_expr(Δs, ∂s, _conj=false, proj=identity)
 
     # Apply `muladd` iteratively.
     # Explicit multiplication is only performed for the first pair of partial and gradient.
-    init_expr = :(*($(_∂s[1]), $(Δs[1])))
-    _∂s_Δs_tail = Iterators.drop(zip(_∂s, Δs), 1)
+    (∂s_1, Δs_1), _∂s_Δs_tail = Iterators.peel(zip(_∂s, Δs))
+    init_expr = :($∂s_1 * $Δs_1)
     summed_∂_mul_Δs = foldl(_∂s_Δs_tail; init=init_expr) do ex, (∂s_i, Δs_i)
         :(muladd($∂s_i, $Δs_i, $ex))
     end