diff --git a/.drone.yml b/.drone.yml
new file mode 100644
index 0000000..e52d282
--- /dev/null
+++ b/.drone.yml
@@ -0,0 +1,27 @@
+---
+kind: pipeline
+type: exec
+name: abacus checks
+
+steps:
+- name: flake check
+  commands:
+  - nix flake check
+
+- name: notifiy
+  commands:
+  - nix run github:ambroisie/matrix-notifier
+  environment:
+    ADDRESS:
+      from_secret: matrix_homeserver
+    ROOM:
+      from_secret: matrix_roomid
+    USER:
+      from_secret: matrix_username
+    PASS:
+      from_secret: matrix_password
+  when:
+    status:
+      - failure
+      - success
+...
diff --git a/.woodpecker/check.yml b/.woodpecker/check.yml
deleted file mode 100644
index 0901624..0000000
--- a/.woodpecker/check.yml
+++ /dev/null
@@ -1,31 +0,0 @@
-labels:
-  type: exec
-
-steps:
-- name: flake check
-  image: bash
-  commands:
-  - nix flake check
-
-- name: package check
-  image: bash
-  commands:
-  - nix build
-
-- name: notify
-  image: bash
-  secrets:
-  - source: matrix_roomid
-    target: room
-  - source: matrix_username
-    target: user
-  - source: matrix_password
-    target: pass
-  - source: matrix_homeserver
-    target: address
-  commands:
-  - nix run github:ambroisie/matrix-notifier
-  when:
-    status:
-    - failure
-    - success
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 7d72bf3..4ef9a7b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -4,15 +4,12 @@ enable_testing()
 
 add_library(common_options INTERFACE)
 target_compile_features(common_options INTERFACE
-  cxx_std_20
+  cxx_std_17
 )
 target_compile_options(common_options INTERFACE
   -Wall
   -Wextra
 )
-target_include_directories(common_options INTERFACE
-  src
-)
 
 add_subdirectory(src)
 add_subdirectory(tests)
diff --git a/flake.nix b/flake.nix
index a697e6b..cdf39b2 100644
--- a/flake.nix
+++ b/flake.nix
@@ -29,40 +29,9 @@
   };
 
   outputs = { self, futils, nixpkgs, pre-commit-hooks }:
-    {
-      overlays.default = final: prev: {
-        abacus = final.stdenv.mkDerivation {
-          pname = "abacus";
-          version = "0.0.0";
-
-          src = self;
-
-          nativeBuildInputs = with final; [
-            bison
-            cmake
-            flex
-            ninja
-            pkg-config
-          ];
-
-          checkInputs = with final; [
-            gtest
-          ];
-
-          doCheck = true;
-
-          meta = with final.lib; {
-            description = "A simple calculator using big numbers";
-            homepage = "https://gitea.belanyi.fr/ambroisie/abacus";
-            license = licenses.mit;
-            maintainers = [ ambroisie ];
-            platforms = platforms.unix;
-          };
-        };
-      };
-    } // futils.lib.eachDefaultSystem (system:
+    futils.lib.eachDefaultSystem (system:
       let
-        pkgs = import nixpkgs { inherit system; overlays = [ self.overlays.default ]; };
+        pkgs = import nixpkgs { inherit system; };
       in
       {
         checks = {
@@ -87,17 +56,46 @@
           };
         };
 
-        devShells.default = pkgs.mkShell {
-          inputsFrom = with self.packages.${system}; [
-            abacus
-          ];
+        defaultPackage = self.packages.${system}.abacus;
+
+        devShell = pkgs.mkShell {
+          inherit (self.defaultPackage.${system})
+            checkInputs
+            nativeBuildInputs
+            ;
 
           inherit (self.checks.${system}.pre-commit) shellHook;
         };
 
         packages = futils.lib.flattenTree {
-          inherit (pkgs) abacus;
-          default = pkgs.abacus;
+          abacus = pkgs.stdenv.mkDerivation {
+            pname = "abacus";
+            version = "0.0.0";
+
+            src = self;
+
+            nativeBuildInputs = with pkgs; [
+              bison
+              cmake
+              flex
+              ninja
+              pkg-config
+            ];
+
+            checkInputs = with pkgs; [
+              gtest
+            ];
+
+            doCheck = true;
+
+            meta = with pkgs.lib; {
+              description = "A simple calculator using big numbers";
+              homepage = "https://gitea.belanyi.fr/ambroisie/abacus";
+              license = licenses.mit;
+              maintainers = [ ambroisie ];
+              platforms = platforms.unix;
+            };
+          };
         };
       });
 }
diff --git a/meson.build b/meson.build
new file mode 100644
index 0000000..4910f6a
--- /dev/null
+++ b/meson.build
@@ -0,0 +1,13 @@
+project(
+  'abacus',
+  'cpp',
+  version: '0.0.0',
+  license: 'MIT',
+  default_options: [
+    'warning_level=3',
+    'cpp_std=c++17',
+  ],
+)
+
+subdir('src')
+subdir('tests')
diff --git a/src/abacus.cc b/src/abacus.cc
index 45d2394..3e735a0 100644
--- a/src/abacus.cc
+++ b/src/abacus.cc
@@ -1,4 +1,4 @@
-#include "parse/parser-driver.hh"
+#include "parser-driver.hh" // FIXME: I would like `parse/parser-driver.hh` path instead...
 
 int main() {
     abacus::parse::ParserDriver driver{};
diff --git a/src/bignum/CMakeLists.txt b/src/bignum/CMakeLists.txt
index 6b64eb6..b2434b7 100644
--- a/src/bignum/CMakeLists.txt
+++ b/src/bignum/CMakeLists.txt
@@ -3,3 +3,7 @@ add_library(bignum STATIC
   bignum.hh
 )
 target_link_libraries(bignum PRIVATE common_options)
+
+target_include_directories(bignum PUBLIC
+  $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>
+)
diff --git a/src/bignum/bignum.cc b/src/bignum/bignum.cc
index f38159f..2121cdc 100644
--- a/src/bignum/bignum.cc
+++ b/src/bignum/bignum.cc
@@ -3,7 +3,6 @@
 #include <algorithm>
 #include <iostream>
 #include <iterator>
-#include <span>
 
 #include <cassert>
 #include <cmath>
@@ -83,17 +82,18 @@ digits_type do_addition(digits_type const& lhs, digits_type const& rhs) {
         ++it2;
     }
 
-    auto leftover = [=]() {
-        if (it1 != end1) {
-            return std::span(it1, end1);
-        }
-        return std::span(it2, end2);
-    }();
+    auto it = it1;
+    auto end = end1;
+    if (it1 == end1) {
+        it = it2;
+        end = end2;
+    }
 
-    for (auto value : leftover) {
-        int addition = value + carry;
+    while (it != end) {
+        int addition = *it + carry;
         carry = addition / BASE;
         res.push_back(addition % BASE);
+        ++it;
     }
 
     if (carry != 0) {
@@ -107,7 +107,7 @@ digits_type do_substraction(digits_type const& lhs, digits_type const& rhs) {
     assert(!do_less_than(lhs, rhs));
 
     digits_type complement;
-    auto const take_complement = [](auto num) { return BASE - 1 - num; };
+    auto const take_complement = [](auto num) { return 9 - num; };
     std::transform(lhs.begin(), lhs.end(), std::back_inserter(complement),
                    take_complement);
 
@@ -363,11 +363,7 @@ bool BigNum::less_than(BigNum const& rhs) const {
         return sign_ < rhs.sign_;
     }
 
-    if (is_positive()) {
-        return do_less_than(digits_, rhs.digits_);
-    } else {
-        return do_less_than(rhs.digits_, digits_);
-    }
+    return do_less_than(digits_, rhs.digits_);
 }
 
 void BigNum::canonicalize() {
@@ -385,15 +381,15 @@ bool BigNum::is_canonicalized() const {
         return sign_ == 0;
     }
 
-    // `back` is valid since there is at least one element
-    auto const has_leading_zero = digits_.back() == 0;
-    if (has_leading_zero) {
+    auto const leading_zeros = std::find_if(digits_.rbegin(), digits_.rend(),
+                                            [](auto v) { return v != 0; });
+    if (leading_zeros != digits_.rbegin()) {
         return false;
     }
 
-    auto const has_overflow = std::any_of(digits_.begin(), digits_.end(),
-                                          [](auto v) { return v >= BASE; });
-    if (has_overflow) {
+    auto const overflow = std::find_if(digits_.begin(), digits_.end(),
+                                       [](auto v) { return v >= BASE; });
+    if (overflow != digits_.end()) {
         return false;
     }
 
diff --git a/src/bignum/meson.build b/src/bignum/meson.build
new file mode 100644
index 0000000..af1e4e5
--- /dev/null
+++ b/src/bignum/meson.build
@@ -0,0 +1,16 @@
+bignum_sources = files(
+  'bignum.cc',
+  'bignum.hh',
+)
+
+bignum_inc = include_directories('.')
+
+bignum_lib = static_library(
+  'bignum',
+  sources: bignum_sources,
+)
+
+bignum = declare_dependency(
+  link_with: bignum_lib,
+  include_directories: bignum_inc,
+)
diff --git a/src/meson.build b/src/meson.build
new file mode 100644
index 0000000..a6c5cb2
--- /dev/null
+++ b/src/meson.build
@@ -0,0 +1,16 @@
+abacus_sources = files(
+  'abacus.cc',
+)
+
+subdir('bignum')
+subdir('parse')
+
+abacus = executable(
+  'abacus',
+  sources: abacus_sources,
+  dependencies: [
+    bignum,
+    parse,
+  ],
+  install: true,
+)
diff --git a/src/parse/CMakeLists.txt b/src/parse/CMakeLists.txt
index 22a706c..3c87a92 100644
--- a/src/parse/CMakeLists.txt
+++ b/src/parse/CMakeLists.txt
@@ -18,6 +18,6 @@ target_link_libraries(parse PRIVATE
 )
 
 target_include_directories(parse PUBLIC
-  $<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}>
   $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>
+  $<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}>
 )
diff --git a/src/parse/meson.build b/src/parse/meson.build
new file mode 100644
index 0000000..1a3bdcb
--- /dev/null
+++ b/src/parse/meson.build
@@ -0,0 +1,48 @@
+flex_binary = find_program('flex', required: true)
+lexer_sources = custom_target(
+  'lexer_sources',
+  input: 'scanner.ll',
+  output: 'scanner.cc',
+  command: [
+    flex_binary,
+    '-o',
+    '@OUTPUT@',
+    '@INPUT@',
+  ],
+)
+
+bison_binary = find_program('bison', required: true)
+parser_sources = custom_target(
+  'parser_sources',
+  input: 'parser.yy',
+  output: [
+    'parser.cc',
+    'parser.hh',
+  ],
+  command: [
+    bison_binary,
+    '@INPUT@',
+    '--output=@OUTPUT0@',
+    '--defines=@OUTPUT1@',
+    '--graph',
+    # FIXME: html output in bison 3.7.90
+  ],
+)
+
+parse_inc = include_directories('.')
+
+parse_lib = static_library(
+  'parser',
+  'parser-driver.cc',
+  'parser-driver.hh',
+  lexer_sources,
+  parser_sources,
+  dependencies: [
+    bignum,
+  ],
+)
+
+parse = declare_dependency(
+  link_with: parse_lib,
+  include_directories: parse_inc,
+)
diff --git a/src/parse/parser-driver.hh b/src/parse/parser-driver.hh
index ad88efd..d5ba7ea 100644
--- a/src/parse/parser-driver.hh
+++ b/src/parse/parser-driver.hh
@@ -4,7 +4,7 @@
 
 #include "parser.hh"
 
-#include "bignum/bignum.hh"
+#include "bignum.hh" // FIXME: I would like `bignum/bignum.hh` path instead...
 
 namespace abacus::parse {
 
diff --git a/src/parse/parser.yy b/src/parse/parser.yy
index 07e3e3a..d8fbc87 100644
--- a/src/parse/parser.yy
+++ b/src/parse/parser.yy
@@ -36,7 +36,7 @@ namespace abacus::parse {
 class ParserDriver;
 } // namespace abacus::parse
 
-#include "bignum/bignum.hh"
+#include "bignum.hh" // FIXME: I would like `bignum/bignum.hh` path instead...
 }
 
 %code provides {
diff --git a/tests/meson.build b/tests/meson.build
new file mode 100644
index 0000000..082b746
--- /dev/null
+++ b/tests/meson.build
@@ -0,0 +1 @@
+subdir('unit')
diff --git a/tests/unit/bignum.cc b/tests/unit/bignum.cc
index 1126a17..f584778 100644
--- a/tests/unit/bignum.cc
+++ b/tests/unit/bignum.cc
@@ -2,7 +2,7 @@
 
 #include <gtest/gtest.h>
 
-#include "bignum/bignum.hh"
+#include "bignum.hh"
 
 using namespace abacus::bignum;
 
@@ -68,34 +68,6 @@ TEST(BigNum, comparisons_digits) {
     EXPECT_GT(ten, nine);
 }
 
-TEST(BigNum, comparisons_negative) {
-    auto const zero = BigNum(0);
-    auto const minus_one = BigNum(-1);
-    auto const minus_two = BigNum(-2);
-
-    EXPECT_LT(minus_one, zero);
-    EXPECT_LE(minus_one, zero);
-    EXPECT_LT(minus_two, minus_one);
-    EXPECT_LE(minus_two, minus_one);
-    EXPECT_LE(minus_one, minus_one);
-    EXPECT_GE(minus_two, minus_two);
-
-    EXPECT_GT(zero, minus_one);
-    EXPECT_GE(zero, minus_one);
-    EXPECT_GT(minus_one, minus_two);
-    EXPECT_GE(minus_one, minus_two);
-    EXPECT_GE(minus_one, minus_one);
-    EXPECT_GE(minus_two, minus_two);
-}
-
-TEST(BigNum, comparisons_digits_negative) {
-    auto const minus_nine = BigNum(-9);
-    auto const minus_ten = BigNum(-10);
-
-    EXPECT_LT(minus_ten, minus_nine);
-    EXPECT_GT(minus_nine, minus_ten);
-}
-
 TEST(BigNum, unary) {
     auto const zero = BigNum(0);
     auto const one = BigNum(1);
diff --git a/tests/unit/meson.build b/tests/unit/meson.build
new file mode 100644
index 0000000..db8eddd
--- /dev/null
+++ b/tests/unit/meson.build
@@ -0,0 +1,26 @@
+gtest = dependency(
+  'gtest',
+  main: true,
+  required: false,
+)
+
+if gtest.found()
+  unit_test_sources = files(
+    'bignum.cc',
+  )
+
+  unit_tests = executable(
+    'unit_tests',
+    sources: unit_test_sources,
+    dependencies: [
+      bignum,
+      gtest,
+    ],
+  )
+
+  test(
+    'unit tests',
+    unit_tests,
+    protocol: 'gtest',
+  )
+endif