diff --git a/overlay.nix b/overlay.nix
index 255f1d2..ddbbaae 100644
--- a/overlay.nix
+++ b/overlay.nix
@@ -1,6 +1,6 @@
 self: super:
 rec {
-   yosys = super.yosys.overrideAttrs(oa: rec {
+  yosys = super.yosys.overrideAttrs(oa: rec {
     name = "yosys-${version}";
     version = "0.9";
     srcs = [
@@ -12,6 +12,9 @@ rec {
         name   = "yosys";
       })
 
+      # NOTE: the version of abc used here is synchronized with
+      # the one in the yosys Makefile of the version above;
+      # keep them the same for quality purposes.
       (super.fetchFromGitHub {
         owner  = "berkeley-abc";
         repo   = "abc";
diff --git a/shell.nix b/shell.nix
index 86d4ff2..79cfb8f 100644
--- a/shell.nix
+++ b/shell.nix
@@ -5,9 +5,9 @@ let
 in
   pkgs.mkShell {
     buildInputs = [
-    	hx.binutils-riscv32 hx.binutils-riscv64 hx.gcc-riscv32 hx.gcc-riscv64 pkgs.rustc pkgs.cargo hx.rust-riscv32i-crates
-    	(pkgs.python3.withPackages(ps: [hx.nmigen hx.nmigen-boards hx.heavycomps hx.minerva]))
-    	pkgs.yosys
+      hx.binutils-riscv32 hx.binutils-riscv64 hx.gcc-riscv32 hx.gcc-riscv64 pkgs.rustc pkgs.cargo hx.rust-riscv32i-crates
+      (pkgs.python3.withPackages(ps: [hx.nmigen hx.nmigen-boards hx.heavycomps hx.minerva]))
+      pkgs.yosys
       pkgs.nextpnr
       pkgs.trellis
       pkgs.openocd