Create a generic BTreeG for Go 1.18 and beyond.

.github/workflows/test.yml

@@ -1,7 +1,7 @@
 on: [push, pull_request]
 name: Test
 jobs:
-  test:
+  test111:
     runs-on: ubuntu-latest
     steps:
     - name: Install Go
@@ -11,4 +11,81 @@ jobs:
     - name: Checkout code
       uses: actions/checkout@v2
     - name: Test
-      run: go test -v ./...
+      run: go test -v ./...
+  test112:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Install Go
+      uses: actions/setup-go@v2
+      with:
+        go-version: 1.12.x
+    - name: Checkout code
+      uses: actions/checkout@v2
+    - name: Test
+      run: go test -v ./...
+  test113:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Install Go
+      uses: actions/setup-go@v2
+      with:
+        go-version: 1.13.x
+    - name: Checkout code
+      uses: actions/checkout@v2
+    - name: Test
+      run: go test -v ./...
+  test114:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Install Go
+      uses: actions/setup-go@v2
+      with:
+        go-version: 1.14.x
+    - name: Checkout code
+      uses: actions/checkout@v2
+    - name: Test
+      run: go test -v ./...
+  test115:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Install Go
+      uses: actions/setup-go@v2
+      with:
+        go-version: 1.15.x
+    - name: Checkout code
+      uses: actions/checkout@v2
+    - name: Test
+      run: go test -v ./...
+  test116:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Install Go
+      uses: actions/setup-go@v2
+      with:
+        go-version: 1.16.x
+    - name: Checkout code
+      uses: actions/checkout@v2
+    - name: Test
+      run: go test -v ./...
+  test117:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Install Go
+      uses: actions/setup-go@v2
+      with:
+        go-version: 1.17.x
+    - name: Checkout code
+      uses: actions/checkout@v2
+    - name: Test
+      run: go test -v ./...
+  test118:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Install Go
+      uses: actions/setup-go@v2
+      with:
+        go-version: 1.18.x
+    - name: Checkout code
+      uses: actions/checkout@v2
+    - name: Test
+      run: go test -v ./...

btree.go

@@ -12,6 +12,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build !go1.18
+// +build !go1.18
+
 // Package btree implements in-memory B-Trees of arbitrary degree.
 //
 // btree implements an in-memory B-Tree for use as an ordered data structure.