From bd64864bb1604039098433a9e46abc11b122d0dd Mon Sep 17 00:00:00 2001
From: Frames White <me@oxinabox.net>
Date: Wed, 7 Feb 2024 10:46:25 +0800
Subject: [PATCH 1/4] Implement Tangent subtraction

---
 src/tangent_arithmetic.jl                |  1 +
 test/tangent_types/structural_tangent.jl | 14 ++++++++++++++
 2 files changed, 15 insertions(+)

diff --git a/src/tangent_arithmetic.jl b/src/tangent_arithmetic.jl
index 18ae7b3ad..f79957311 100644
--- a/src/tangent_arithmetic.jl
+++ b/src/tangent_arithmetic.jl
@@ -146,6 +146,7 @@ Base.:+(a::Dict, d::Tangent{P}) where {P} = merge(+, a, backing(d))
 Base.:+(a::StructuralTangent{P}, b::P) where {P} = b + a
 
 Base.:-(tangent::StructuralTangent{P}) where {P} = map(-, tangent)
+Base.:-(a::StructuralTangent{P}, b::StructuralTangent{P}) where {P} = a + (-b)
 
 # We intentionally do not define, `Base.*(::Tangent, ::Tangent)` as that is not meaningful
 # In general one doesn't have to represent multiplications of 2 tangents
diff --git a/test/tangent_types/structural_tangent.jl b/test/tangent_types/structural_tangent.jl
index c177b05f4..0cd06a16d 100644
--- a/test/tangent_types/structural_tangent.jl
+++ b/test/tangent_types/structural_tangent.jl
@@ -358,6 +358,20 @@ end
             @test -1.0 * t == -t
         end
 
+        @test "subtraction" begin
+            a = Tangent{Foo}(; x=2.0, y=-2.0)
+            b = Tangent{Foo}(; x=1.0, y=2.0)
+            @test (a - b) == Tangent{Foo}(; x=1.0, y=-4.0)
+
+            a = Tangent{Foo}(; x=2.0, y=-2.0)
+            b = Tangent{Foo}(; x=1.0)
+            @test (a - b) == Tangent{Foo}(; x=1.0, y=-2.0)
+            
+            a = Tangent{Tuple{Float64, Float64}}(2.0, 3.0)
+            b = Tangent{Tuple{Float64, Float64}}(1.0, 1.0)
+            @test (a - b) == Tangent{Tuple{Float64, Float64}}(1.0, 2.0)
+        end
+
         @testset "scaling" begin
             @test (
                 2 * Tangent{Foo}(; y=1.5, x=2.5) ==

From a52a213576d50701e2a98fe30d60bb7baae334a9 Mon Sep 17 00:00:00 2001
From: Frames White <me@oxinabox.net>
Date: Wed, 7 Feb 2024 10:50:14 +0800
Subject: [PATCH 2/4] Also test for mutable tangent

---
 test/tangent_types/structural_tangent.jl | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/test/tangent_types/structural_tangent.jl b/test/tangent_types/structural_tangent.jl
index 0cd06a16d..40b1e2a56 100644
--- a/test/tangent_types/structural_tangent.jl
+++ b/test/tangent_types/structural_tangent.jl
@@ -370,6 +370,11 @@ end
             a = Tangent{Tuple{Float64, Float64}}(2.0, 3.0)
             b = Tangent{Tuple{Float64, Float64}}(1.0, 1.0)
             @test (a - b) == Tangent{Tuple{Float64, Float64}}(1.0, 2.0)
+
+            a = MutableTangent{MFoo}(x=1.5, y=1.5)
+            b = MutableTangent{MFoo}(x=0.5, y=0.5)
+            @test (a - b) == MutableTangent{MFoo}(x=1.0, y=1.0)
+            
         end
 
         @testset "scaling" begin

From 7230e9403c250c56081a923103e2852664d73586 Mon Sep 17 00:00:00 2001
From: Frames White <oxinabox@ucc.asn.au>
Date: Wed, 7 Feb 2024 10:52:15 +0800
Subject: [PATCH 3/4] formatting

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
---
 test/tangent_types/structural_tangent.jl | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/test/tangent_types/structural_tangent.jl b/test/tangent_types/structural_tangent.jl
index 40b1e2a56..807c32a33 100644
--- a/test/tangent_types/structural_tangent.jl
+++ b/test/tangent_types/structural_tangent.jl
@@ -366,15 +366,14 @@ end
             a = Tangent{Foo}(; x=2.0, y=-2.0)
             b = Tangent{Foo}(; x=1.0)
             @test (a - b) == Tangent{Foo}(; x=1.0, y=-2.0)
-            
-            a = Tangent{Tuple{Float64, Float64}}(2.0, 3.0)
-            b = Tangent{Tuple{Float64, Float64}}(1.0, 1.0)
-            @test (a - b) == Tangent{Tuple{Float64, Float64}}(1.0, 2.0)
 
-            a = MutableTangent{MFoo}(x=1.5, y=1.5)
-            b = MutableTangent{MFoo}(x=0.5, y=0.5)
-            @test (a - b) == MutableTangent{MFoo}(x=1.0, y=1.0)
-            
+            a = Tangent{Tuple{Float64,Float64}}(2.0, 3.0)
+            b = Tangent{Tuple{Float64,Float64}}(1.0, 1.0)
+            @test (a - b) == Tangent{Tuple{Float64,Float64}}(1.0, 2.0)
+
+            a = MutableTangent{MFoo}(; x=1.5, y=1.5)
+            b = MutableTangent{MFoo}(; x=0.5, y=0.5)
+            @test (a - b) == MutableTangent{MFoo}(; x=1.0, y=1.0)
         end
 
         @testset "scaling" begin

From c7e00c7380910b444e66d8fa3ee7313133032502 Mon Sep 17 00:00:00 2001
From: Frames White <me@oxinabox.net>
Date: Wed, 7 Feb 2024 11:42:18 +0800
Subject: [PATCH 4/4] testset not test

---
 test/tangent_types/structural_tangent.jl | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/test/tangent_types/structural_tangent.jl b/test/tangent_types/structural_tangent.jl
index 807c32a33..d93487703 100644
--- a/test/tangent_types/structural_tangent.jl
+++ b/test/tangent_types/structural_tangent.jl
@@ -358,7 +358,7 @@ end
             @test -1.0 * t == -t
         end
 
-        @test "subtraction" begin
+        @testset "subtraction" begin
             a = Tangent{Foo}(; x=2.0, y=-2.0)
             b = Tangent{Foo}(; x=1.0, y=2.0)
             @test (a - b) == Tangent{Foo}(; x=1.0, y=-4.0)