diff --git a/arion-compose.cabal b/arion-compose.cabal
index 83aa6f6..2ff2f0b 100644
--- a/arion-compose.cabal
+++ b/arion-compose.cabal
@@ -19,6 +19,7 @@ data-files:          nix/*.nix
                    , nix/modules/composition/*.nix
                    , nix/modules/nixos/*.nix
                    , nix/modules/service/*.nix
+                   , nix/modules/lib/*.nix
 
 -- all data is verbatim from some sources
 data-dir:            src
diff --git a/nix/arion.nix b/nix/arion.nix
index 4b76fc1..b53b1f4 100644
--- a/nix/arion.nix
+++ b/nix/arion.nix
@@ -11,7 +11,7 @@ let
   eval = import (srcDir + "/nix/eval-composition.nix");
   build = args@{...}:
     let composition = eval args;
-    in composition.config.build.dockerComposeYaml;
+    in composition.config.out.dockerComposeYaml;
 
 in
   justStaticExecutables (overrideCabal arion-compose (o: {
diff --git a/src/haskell/lib/Arion/Nix.hs b/src/haskell/lib/Arion/Nix.hs
index cef6a92..a90343a 100644
--- a/src/haskell/lib/Arion/Nix.hs
+++ b/src/haskell/lib/Arion/Nix.hs
@@ -51,7 +51,7 @@ evaluateComposition ea = do
         , "--strict"
         , "--json"
         , "--attr"
-        , "config.build.dockerComposeYamlAttrs"
+        , "config.out.dockerComposeYamlAttrs"
         ]
       args =
         [ evalComposition ]
@@ -99,7 +99,7 @@ buildComposition outLink ea = do
   evalComposition <- getEvalCompositionFile
   let commandArgs =
         [ "--attr"
-        , "config.build.dockerComposeYaml"
+        , "config.out.dockerComposeYaml"
         , "--out-link"
         , outLink
         ]
diff --git a/src/nix/eval-composition.nix b/src/nix/eval-composition.nix
index c890973..1081864 100644
--- a/src/nix/eval-composition.nix
+++ b/src/nix/eval-composition.nix
@@ -34,7 +34,7 @@ let
   };
 
 in
-  # Typically you need composition.config.build.dockerComposeYaml
+  # Typically you need composition.config.out.dockerComposeYaml
   composition // {
     # throw in lib and pkgs for repl convenience
     inherit lib;
diff --git a/src/nix/modules/composition/arion-base-image.nix b/src/nix/modules/composition/arion-base-image.nix
index 2be1d97..132bc4c 100644
--- a/src/nix/modules/composition/arion-base-image.nix
+++ b/src/nix/modules/composition/arion-base-image.nix
@@ -34,7 +34,7 @@ in
 
   config = {
     arionBaseImage = "${name}:${tag}";
-    build.imagesToLoad = lib.mkIf (lib.any (s: s.service.useHostStore) (lib.attrValues config.docker-compose.services)) [
+    build.imagesToLoad = lib.mkIf (lib.any (s: s.service.useHostStore) (lib.attrValues config.services)) [
       { image = builtImage; imageName = name; imageTag = tag; }
     ];  
   };
diff --git a/src/nix/modules/composition/docker-compose.nix b/src/nix/modules/composition/docker-compose.nix
index 577fbb8..a74c958 100644
--- a/src/nix/modules/composition/docker-compose.nix
+++ b/src/nix/modules/composition/docker-compose.nix
@@ -3,10 +3,10 @@
    This is a composition-level module.
 
    It defines the low-level options that are read by arion, like
-    - build.dockerComposeYaml
+    - out.dockerComposeYaml
 
    It declares options like
-    - docker-compose.services
+    - services
 
  */
 compositionArgs@{ lib, config, options, pkgs, ... }:
@@ -31,20 +31,24 @@ let
 
 in
 {
+  imports = [
+    ../lib/assert.nix
+    (lib.mkRenamedOptionModule ["docker-compose" "services"] ["services"])
+  ];
   options = {
-    build.dockerComposeYaml = lib.mkOption {
+    out.dockerComposeYaml = lib.mkOption {
       type = lib.types.package;
       description = "A derivation that produces a docker-compose.yaml file for this composition.";
       readOnly = true;
     };
-    build.dockerComposeYamlText = lib.mkOption {
+    out.dockerComposeYamlText = lib.mkOption {
       type = lib.types.str;
-      description = "The text of build.dockerComposeYaml.";
+      description = "The text of out.dockerComposeYaml.";
       readOnly = true;
     };
-    build.dockerComposeYamlAttrs = lib.mkOption {
+    out.dockerComposeYamlAttrs = lib.mkOption {
       type = lib.types.attrsOf lib.types.unspecified;
-      description = "The text of build.dockerComposeYaml.";
+      description = "The text of out.dockerComposeYaml.";
       readOnly = true;
     };
     docker-compose.raw = lib.mkOption {
@@ -55,19 +59,19 @@ in
       type = lib.types.attrs;
       description = "Attribute set that will be turned into the x-arion section of the docker-compose.yaml file.";
     };
-    docker-compose.services = lib.mkOption {
+    services = lib.mkOption {
       type = lib.types.attrsOf (lib.types.submodule service);
       description = "An attribute set of service configurations. A service specifies how to run an image as a container.";
     };
   };
   config = {
-    build.dockerComposeYaml = pkgs.writeText "docker-compose.yaml" config.build.dockerComposeYamlText;
-    build.dockerComposeYamlText = builtins.toJSON (config.build.dockerComposeYamlAttrs);
-    build.dockerComposeYamlAttrs = config.docker-compose.raw;
+    out.dockerComposeYaml = pkgs.writeText "docker-compose.yaml" config.out.dockerComposeYamlText;
+    out.dockerComposeYamlText = builtins.toJSON (config.out.dockerComposeYamlAttrs);
+    out.dockerComposeYamlAttrs = config.assertWarn config.docker-compose.raw;
 
     docker-compose.raw = {
       version = "3.4";
-      services = lib.mapAttrs (k: c: c.build.service) config.docker-compose.services;
+      services = lib.mapAttrs (k: c: c.out.service) config.services;
       x-arion = config.docker-compose.extended;
     };
   };
diff --git a/src/nix/modules/composition/images.nix b/src/nix/modules/composition/images.nix
index bb505da..dc885a2 100644
--- a/src/nix/modules/composition/images.nix
+++ b/src/nix/modules/composition/images.nix
@@ -4,7 +4,7 @@ let
 
   serviceImages =
     lib.mapAttrs addDetails (
-      lib.filterAttrs filterFunction config.docker-compose.services
+      lib.filterAttrs filterFunction config.services
     );
 
   filterFunction = serviceName: service:
@@ -14,7 +14,7 @@ let
   addDetails = serviceName: service:
     builtins.addErrorContext "while evaluating the image for service ${serviceName}"
     (let
-      inherit (service.config) build;
+      inherit (service) build;
     in {
       image = build.image.outPath;
       imageName = build.imageName or service.image.name;
diff --git a/src/nix/modules/composition/service-info.nix b/src/nix/modules/composition/service-info.nix
index 10acb06..4ac106f 100644
--- a/src/nix/modules/composition/service-info.nix
+++ b/src/nix/modules/composition/service-info.nix
@@ -7,7 +7,7 @@
 let
   serviceInfo =
     lib.mapAttrs getInfo (
-      lib.filterAttrs filterFunction config.docker-compose.services
+      lib.filterAttrs filterFunction config.services
     );
 
   filterFunction = _serviceName: service:
diff --git a/src/nix/modules/lib/README.md b/src/nix/modules/lib/README.md
new file mode 100644
index 0000000..48a0c04
--- /dev/null
+++ b/src/nix/modules/lib/README.md
@@ -0,0 +1,2 @@
+`assertions.nix`: Nixpkgs
+`assert.nix`: extracted from Nixpkgs, adapted
diff --git a/src/nix/modules/lib/assert.nix b/src/nix/modules/lib/assert.nix
new file mode 100644
index 0000000..f9bf4a8
--- /dev/null
+++ b/src/nix/modules/lib/assert.nix
@@ -0,0 +1,32 @@
+{ config, lib, pkgs, ... }:
+
+# based on nixpkgs/nixos/modules/system/activation/top-level.nix
+
+let
+  inherit (lib) filter concatStringsSep types mkOption;
+
+  # lib.showWarnings since 19.09
+  showWarnings = warnings: res: lib.fold (w: x: lib.warn w x) res warnings;
+  warn = msg: builtins.trace "warning: ${msg}";
+
+  # Handle assertions and warnings
+
+  failedAssertions = map (x: x.message) (filter (x: !x.assertion) config.assertions);
+
+  assertWarn = if failedAssertions != []
+    then throw "\nFailed assertions:\n${concatStringsSep "\n" (map (x: "- ${x}") failedAssertions)}"
+    else showWarnings config.warnings;
+
+in
+
+{
+  imports = [ ./assertions.nix ];
+  options.assertWarn = mkOption {
+    type = types.unspecified; # a function
+    # It's for the wrapping program to know about this. User need not care.
+    internal = true;
+    readOnly = true;
+  };
+  config = { inherit assertWarn; };
+}
+
diff --git a/src/nix/modules/lib/assertions.nix b/src/nix/modules/lib/assertions.nix
new file mode 100644
index 0000000..550b3ac
--- /dev/null
+++ b/src/nix/modules/lib/assertions.nix
@@ -0,0 +1,34 @@
+{ lib, ... }:
+
+with lib;
+
+{
+
+  options = {
+
+    assertions = mkOption {
+      type = types.listOf types.unspecified;
+      internal = true;
+      default = [];
+      example = [ { assertion = false; message = "you can't enable this for that reason"; } ];
+      description = ''
+        This option allows modules to express conditions that must
+        hold for the evaluation of the system configuration to
+        succeed, along with associated error messages for the user.
+      '';
+    };
+
+    warnings = mkOption {
+      internal = true;
+      default = [];
+      type = types.listOf types.str;
+      example = [ "The `foo' service is deprecated and will go away soon!" ];
+      description = ''
+        This option allows modules to show warnings to users during
+        the evaluation of the system configuration.
+      '';
+    };
+
+  };
+  # impl of assertions is in <nixpkgs/nixos/modules/system/activation/top-level.nix>
+}
diff --git a/src/nix/modules/service/all-modules.nix b/src/nix/modules/service/all-modules.nix
index bfbb13c..18c0432 100644
--- a/src/nix/modules/service/all-modules.nix
+++ b/src/nix/modules/service/all-modules.nix
@@ -7,4 +7,5 @@
   ./image.nix
   ./nixos.nix
   ./nixos-init.nix
+  ../lib/assert.nix
 ]
diff --git a/src/nix/modules/service/docker-compose-service.nix b/src/nix/modules/service/docker-compose-service.nix
index 80a795d..0451b78 100644
--- a/src/nix/modules/service/docker-compose-service.nix
+++ b/src/nix/modules/service/docker-compose-service.nix
@@ -1,6 +1,6 @@
 /*
 
-   This service-level module defines the build.service option, using
+   This service-level module defines the out.service option, using
    the user-facing options service.image, service.volumes, etc.
 
  */
@@ -25,8 +25,12 @@ let
 
 in
 {
+  imports = [
+    (lib.mkRenamedOptionModule ["build" "service"] ["out" "service"])
+  ];
+
   options = {
-    build.service = mkOption {
+    out.service = mkOption {
       type = attrsOf types.unspecified;
       description = ''
         Raw input for the service in <code>docker-compose.yaml</code>.
@@ -37,12 +41,13 @@ in
         This option is user accessible because it may serve as an
         escape hatch for some.
       '';
+      apply = config.assertWarn;
     };
 
     service.name = mkOption {
       type = str;
       description = ''
-        The name of the service - <code>&lt;name></code> in the composition-level <code>docker-compose.services.&lt;name></code>
+        The name of the service - <code>&lt;name></code> in the composition-level <code>services.&lt;name></code>
       '';
       readOnly = true;
     };
@@ -209,7 +214,7 @@ in
     };
   };
 
-  config.build.service = {
+  config.out.service = {
     inherit (config.service)
       volumes
       environment
diff --git a/tests/arion-test/default.nix b/tests/arion-test/default.nix
index e197fb2..caf892a 100644
--- a/tests/arion-test/default.nix
+++ b/tests/arion-test/default.nix
@@ -30,9 +30,9 @@ in
     virtualisation.pathsInNixDB = [
       # Pre-build the image because we don't want to build the world
       # in the vm.
-      (preEval [ ../../examples/minimal/arion-compose.nix ]).config.build.dockerComposeYaml
-      (preEval [ ../../examples/full-nixos/arion-compose.nix ]).config.build.dockerComposeYaml
-      (preEval [ ../../examples/nixos-unit/arion-compose.nix ]).config.build.dockerComposeYaml
+      (preEval [ ../../examples/minimal/arion-compose.nix ]).config.out.dockerComposeYaml
+      (preEval [ ../../examples/full-nixos/arion-compose.nix ]).config.out.dockerComposeYaml
+      (preEval [ ../../examples/nixos-unit/arion-compose.nix ]).config.out.dockerComposeYaml
       pkgs.stdenv
     ];
   };