diff --git a/.github/workflows/nix-action-8.17.yml b/.github/workflows/nix-action-8.17.yml
index f5b74031..6ac225b7 100644
--- a/.github/workflows/nix-action-8.17.yml
+++ b/.github/workflows/nix-action-8.17.yml
@@ -1999,6 +1999,83 @@ jobs:
       name: Building/fetching current CI target
       run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "8.17" --argstr
         job "gappalib"
+  graph-theory:
+    needs:
+    - coq
+    - mathcomp-algebra
+    - mathcomp-finmap
+    - mathcomp-fingroup
+    - fourcolor
+    - hierarchy-builder
+    runs-on: ubuntu-latest
+    steps:
+    - name: Determine which commit to initially checkout
+      run: "if [ ${{ github.event_name }} = \"push\" ]; then\n  echo \"target_commit=${{\
+        \ github.sha }}\" >> $GITHUB_ENV\nelse\n  echo \"target_commit=${{ github.event.pull_request.head.sha\
+        \ }}\" >> $GITHUB_ENV\nfi\n"
+    - name: Git checkout
+      uses: actions/checkout@v3
+      with:
+        fetch-depth: 0
+        ref: ${{ env.target_commit }}
+    - name: Determine which commit to test
+      run: "if [ ${{ github.event_name }} = \"push\" ]; then\n  echo \"tested_commit=${{\
+        \ github.sha }}\" >> $GITHUB_ENV\nelse\n  merge_commit=$(git ls-remote ${{\
+        \ github.event.repository.html_url }} refs/pull/${{ github.event.number }}/merge\
+        \ | cut -f1)\n  mergeable=$(git merge --no-commit --no-ff ${{ github.event.pull_request.base.sha\
+        \ }} > /dev/null 2>&1; echo $?; git merge --abort > /dev/null 2>&1 || true)\n\
+        \  if [ -z \"$merge_commit\" -o \"x$mergeable\" != \"x0\" ]; then\n    echo\
+        \ \"tested_commit=${{ github.event.pull_request.head.sha }}\" >> $GITHUB_ENV\n\
+        \  else\n    echo \"tested_commit=$merge_commit\" >> $GITHUB_ENV\n  fi\nfi\n"
+    - name: Git checkout
+      uses: actions/checkout@v3
+      with:
+        fetch-depth: 0
+        ref: ${{ env.tested_commit }}
+    - name: Cachix install
+      uses: cachix/install-nix-action@v20
+      with:
+        nix_path: nixpkgs=channel:nixpkgs-unstable
+    - name: Cachix setup coq-community
+      uses: cachix/cachix-action@v12
+      with:
+        authToken: ${{ secrets.CACHIX_AUTH_TOKEN }}
+        extraPullNames: coq, math-comp
+        name: coq-community
+    - id: stepCheck
+      name: Checking presence of CI target graph-theory
+      run: "nb_dry_run=$(NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link \\\n   --argstr\
+        \ bundle \"8.17\" --argstr job \"graph-theory\" \\\n   --dry-run 2>&1 > /dev/null)\n\
+        echo $nb_dry_run\necho status=$(echo $nb_dry_run | grep \"built:\" | sed \"\
+        s/.*/built/\") >> $GITHUB_OUTPUT\n"
+    - if: steps.stepCheck.outputs.status == 'built'
+      name: 'Building/fetching previous CI target: coq'
+      run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "8.17" --argstr
+        job "coq"
+    - if: steps.stepCheck.outputs.status == 'built'
+      name: 'Building/fetching previous CI target: mathcomp-algebra'
+      run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "8.17" --argstr
+        job "mathcomp-algebra"
+    - if: steps.stepCheck.outputs.status == 'built'
+      name: 'Building/fetching previous CI target: mathcomp-finmap'
+      run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "8.17" --argstr
+        job "mathcomp-finmap"
+    - if: steps.stepCheck.outputs.status == 'built'
+      name: 'Building/fetching previous CI target: mathcomp-fingroup'
+      run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "8.17" --argstr
+        job "mathcomp-fingroup"
+    - if: steps.stepCheck.outputs.status == 'built'
+      name: 'Building/fetching previous CI target: fourcolor'
+      run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "8.17" --argstr
+        job "fourcolor"
+    - if: steps.stepCheck.outputs.status == 'built'
+      name: 'Building/fetching previous CI target: hierarchy-builder'
+      run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "8.17" --argstr
+        job "hierarchy-builder"
+    - if: steps.stepCheck.outputs.status == 'built'
+      name: Building/fetching current CI target
+      run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "8.17" --argstr
+        job "graph-theory"
   hierarchy-builder:
     needs:
     - coq
@@ -3284,68 +3361,6 @@ jobs:
       name: Building/fetching current CI target
       run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "8.17" --argstr
         job "mathcomp-ssreflect"
-  mathcomp-tarjan:
-    needs:
-    - coq
-    - mathcomp-ssreflect
-    - mathcomp-fingroup
-    runs-on: ubuntu-latest
-    steps:
-    - name: Determine which commit to initially checkout
-      run: "if [ ${{ github.event_name }} = \"push\" ]; then\n  echo \"target_commit=${{\
-        \ github.sha }}\" >> $GITHUB_ENV\nelse\n  echo \"target_commit=${{ github.event.pull_request.head.sha\
-        \ }}\" >> $GITHUB_ENV\nfi\n"
-    - name: Git checkout
-      uses: actions/checkout@v3
-      with:
-        fetch-depth: 0
-        ref: ${{ env.target_commit }}
-    - name: Determine which commit to test
-      run: "if [ ${{ github.event_name }} = \"push\" ]; then\n  echo \"tested_commit=${{\
-        \ github.sha }}\" >> $GITHUB_ENV\nelse\n  merge_commit=$(git ls-remote ${{\
-        \ github.event.repository.html_url }} refs/pull/${{ github.event.number }}/merge\
-        \ | cut -f1)\n  mergeable=$(git merge --no-commit --no-ff ${{ github.event.pull_request.base.sha\
-        \ }} > /dev/null 2>&1; echo $?; git merge --abort > /dev/null 2>&1 || true)\n\
-        \  if [ -z \"$merge_commit\" -o \"x$mergeable\" != \"x0\" ]; then\n    echo\
-        \ \"tested_commit=${{ github.event.pull_request.head.sha }}\" >> $GITHUB_ENV\n\
-        \  else\n    echo \"tested_commit=$merge_commit\" >> $GITHUB_ENV\n  fi\nfi\n"
-    - name: Git checkout
-      uses: actions/checkout@v3
-      with:
-        fetch-depth: 0
-        ref: ${{ env.tested_commit }}
-    - name: Cachix install
-      uses: cachix/install-nix-action@v20
-      with:
-        nix_path: nixpkgs=channel:nixpkgs-unstable
-    - name: Cachix setup coq-community
-      uses: cachix/cachix-action@v12
-      with:
-        authToken: ${{ secrets.CACHIX_AUTH_TOKEN }}
-        extraPullNames: coq, math-comp
-        name: coq-community
-    - id: stepCheck
-      name: Checking presence of CI target mathcomp-tarjan
-      run: "nb_dry_run=$(NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link \\\n   --argstr\
-        \ bundle \"8.17\" --argstr job \"mathcomp-tarjan\" \\\n   --dry-run 2>&1 >\
-        \ /dev/null)\necho $nb_dry_run\necho status=$(echo $nb_dry_run | grep \"built:\"\
-        \ | sed \"s/.*/built/\") >> $GITHUB_OUTPUT\n"
-    - if: steps.stepCheck.outputs.status == 'built'
-      name: 'Building/fetching previous CI target: coq'
-      run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "8.17" --argstr
-        job "coq"
-    - if: steps.stepCheck.outputs.status == 'built'
-      name: 'Building/fetching previous CI target: mathcomp-ssreflect'
-      run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "8.17" --argstr
-        job "mathcomp-ssreflect"
-    - if: steps.stepCheck.outputs.status == 'built'
-      name: 'Building/fetching previous CI target: mathcomp-fingroup'
-      run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "8.17" --argstr
-        job "mathcomp-fingroup"
-    - if: steps.stepCheck.outputs.status == 'built'
-      name: Building/fetching current CI target
-      run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "8.17" --argstr
-        job "mathcomp-tarjan"
   mathcomp-word:
     needs:
     - coq
diff --git a/.github/workflows/nix-action-8.18.yml b/.github/workflows/nix-action-8.18.yml
index c17fc5b1..b63cfcdc 100644
--- a/.github/workflows/nix-action-8.18.yml
+++ b/.github/workflows/nix-action-8.18.yml
@@ -1580,6 +1580,83 @@ jobs:
       name: Building/fetching current CI target
       run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "8.18" --argstr
         job "fourcolor"
+  graph-theory:
+    needs:
+    - coq
+    - mathcomp-algebra
+    - mathcomp-finmap
+    - mathcomp-fingroup
+    - fourcolor
+    - hierarchy-builder
+    runs-on: ubuntu-latest
+    steps:
+    - name: Determine which commit to initially checkout
+      run: "if [ ${{ github.event_name }} = \"push\" ]; then\n  echo \"target_commit=${{\
+        \ github.sha }}\" >> $GITHUB_ENV\nelse\n  echo \"target_commit=${{ github.event.pull_request.head.sha\
+        \ }}\" >> $GITHUB_ENV\nfi\n"
+    - name: Git checkout
+      uses: actions/checkout@v3
+      with:
+        fetch-depth: 0
+        ref: ${{ env.target_commit }}
+    - name: Determine which commit to test
+      run: "if [ ${{ github.event_name }} = \"push\" ]; then\n  echo \"tested_commit=${{\
+        \ github.sha }}\" >> $GITHUB_ENV\nelse\n  merge_commit=$(git ls-remote ${{\
+        \ github.event.repository.html_url }} refs/pull/${{ github.event.number }}/merge\
+        \ | cut -f1)\n  mergeable=$(git merge --no-commit --no-ff ${{ github.event.pull_request.base.sha\
+        \ }} > /dev/null 2>&1; echo $?; git merge --abort > /dev/null 2>&1 || true)\n\
+        \  if [ -z \"$merge_commit\" -o \"x$mergeable\" != \"x0\" ]; then\n    echo\
+        \ \"tested_commit=${{ github.event.pull_request.head.sha }}\" >> $GITHUB_ENV\n\
+        \  else\n    echo \"tested_commit=$merge_commit\" >> $GITHUB_ENV\n  fi\nfi\n"
+    - name: Git checkout
+      uses: actions/checkout@v3
+      with:
+        fetch-depth: 0
+        ref: ${{ env.tested_commit }}
+    - name: Cachix install
+      uses: cachix/install-nix-action@v20
+      with:
+        nix_path: nixpkgs=channel:nixpkgs-unstable
+    - name: Cachix setup coq-community
+      uses: cachix/cachix-action@v12
+      with:
+        authToken: ${{ secrets.CACHIX_AUTH_TOKEN }}
+        extraPullNames: coq, math-comp
+        name: coq-community
+    - id: stepCheck
+      name: Checking presence of CI target graph-theory
+      run: "nb_dry_run=$(NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link \\\n   --argstr\
+        \ bundle \"8.18\" --argstr job \"graph-theory\" \\\n   --dry-run 2>&1 > /dev/null)\n\
+        echo $nb_dry_run\necho status=$(echo $nb_dry_run | grep \"built:\" | sed \"\
+        s/.*/built/\") >> $GITHUB_OUTPUT\n"
+    - if: steps.stepCheck.outputs.status == 'built'
+      name: 'Building/fetching previous CI target: coq'
+      run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "8.18" --argstr
+        job "coq"
+    - if: steps.stepCheck.outputs.status == 'built'
+      name: 'Building/fetching previous CI target: mathcomp-algebra'
+      run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "8.18" --argstr
+        job "mathcomp-algebra"
+    - if: steps.stepCheck.outputs.status == 'built'
+      name: 'Building/fetching previous CI target: mathcomp-finmap'
+      run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "8.18" --argstr
+        job "mathcomp-finmap"
+    - if: steps.stepCheck.outputs.status == 'built'
+      name: 'Building/fetching previous CI target: mathcomp-fingroup'
+      run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "8.18" --argstr
+        job "mathcomp-fingroup"
+    - if: steps.stepCheck.outputs.status == 'built'
+      name: 'Building/fetching previous CI target: fourcolor'
+      run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "8.18" --argstr
+        job "fourcolor"
+    - if: steps.stepCheck.outputs.status == 'built'
+      name: 'Building/fetching previous CI target: hierarchy-builder'
+      run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "8.18" --argstr
+        job "hierarchy-builder"
+    - if: steps.stepCheck.outputs.status == 'built'
+      name: Building/fetching current CI target
+      run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "8.18" --argstr
+        job "graph-theory"
   hierarchy-builder:
     needs:
     - coq
@@ -2865,68 +2942,6 @@ jobs:
       name: Building/fetching current CI target
       run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "8.18" --argstr
         job "mathcomp-ssreflect"
-  mathcomp-tarjan:
-    needs:
-    - coq
-    - mathcomp-ssreflect
-    - mathcomp-fingroup
-    runs-on: ubuntu-latest
-    steps:
-    - name: Determine which commit to initially checkout
-      run: "if [ ${{ github.event_name }} = \"push\" ]; then\n  echo \"target_commit=${{\
-        \ github.sha }}\" >> $GITHUB_ENV\nelse\n  echo \"target_commit=${{ github.event.pull_request.head.sha\
-        \ }}\" >> $GITHUB_ENV\nfi\n"
-    - name: Git checkout
-      uses: actions/checkout@v3
-      with:
-        fetch-depth: 0
-        ref: ${{ env.target_commit }}
-    - name: Determine which commit to test
-      run: "if [ ${{ github.event_name }} = \"push\" ]; then\n  echo \"tested_commit=${{\
-        \ github.sha }}\" >> $GITHUB_ENV\nelse\n  merge_commit=$(git ls-remote ${{\
-        \ github.event.repository.html_url }} refs/pull/${{ github.event.number }}/merge\
-        \ | cut -f1)\n  mergeable=$(git merge --no-commit --no-ff ${{ github.event.pull_request.base.sha\
-        \ }} > /dev/null 2>&1; echo $?; git merge --abort > /dev/null 2>&1 || true)\n\
-        \  if [ -z \"$merge_commit\" -o \"x$mergeable\" != \"x0\" ]; then\n    echo\
-        \ \"tested_commit=${{ github.event.pull_request.head.sha }}\" >> $GITHUB_ENV\n\
-        \  else\n    echo \"tested_commit=$merge_commit\" >> $GITHUB_ENV\n  fi\nfi\n"
-    - name: Git checkout
-      uses: actions/checkout@v3
-      with:
-        fetch-depth: 0
-        ref: ${{ env.tested_commit }}
-    - name: Cachix install
-      uses: cachix/install-nix-action@v20
-      with:
-        nix_path: nixpkgs=channel:nixpkgs-unstable
-    - name: Cachix setup coq-community
-      uses: cachix/cachix-action@v12
-      with:
-        authToken: ${{ secrets.CACHIX_AUTH_TOKEN }}
-        extraPullNames: coq, math-comp
-        name: coq-community
-    - id: stepCheck
-      name: Checking presence of CI target mathcomp-tarjan
-      run: "nb_dry_run=$(NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link \\\n   --argstr\
-        \ bundle \"8.18\" --argstr job \"mathcomp-tarjan\" \\\n   --dry-run 2>&1 >\
-        \ /dev/null)\necho $nb_dry_run\necho status=$(echo $nb_dry_run | grep \"built:\"\
-        \ | sed \"s/.*/built/\") >> $GITHUB_OUTPUT\n"
-    - if: steps.stepCheck.outputs.status == 'built'
-      name: 'Building/fetching previous CI target: coq'
-      run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "8.18" --argstr
-        job "coq"
-    - if: steps.stepCheck.outputs.status == 'built'
-      name: 'Building/fetching previous CI target: mathcomp-ssreflect'
-      run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "8.18" --argstr
-        job "mathcomp-ssreflect"
-    - if: steps.stepCheck.outputs.status == 'built'
-      name: 'Building/fetching previous CI target: mathcomp-fingroup'
-      run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "8.18" --argstr
-        job "mathcomp-fingroup"
-    - if: steps.stepCheck.outputs.status == 'built'
-      name: Building/fetching current CI target
-      run: NIXPKGS_ALLOW_UNFREE=1 nix-build --no-out-link --argstr bundle "8.18" --argstr
-        job "mathcomp-tarjan"
   mathcomp-word:
     needs:
     - coq
diff --git a/.nix/nixpkgs.nix b/.nix/nixpkgs.nix
index 85e369ef..23f9c534 100644
--- a/.nix/nixpkgs.nix
+++ b/.nix/nixpkgs.nix
@@ -1,4 +1,4 @@
 fetchTarball {
-    url = https://github.com/NixOS/nixpkgs/archive/e06c716ef149f466f25e62a836c30b90476e65e5.tar.gz;
-    sha256 = "0a8pxwnjddqliswy54f5c6hzw1xip4rpcrgd0222krb49pr39zgm";
+    url = https://github.com/NixOS/nixpkgs/archive/b28c9ac0a4ecb8b978e71555ca52e604b5802e2d.tar.gz;
+    sha256 = "05dw50ns1gq6mx59pqbgp1ail314jrh52vz8300m5zyvlklxb5v9";
   }