diff --git a/package.json b/package.json
index 61d6ccc..b07df47 100644
--- a/package.json
+++ b/package.json
@@ -37,7 +37,7 @@
   },
   "jest": {
     "testMatch": [
-      "<rootDir>/tests/**"
+      "<rootDir>/tests/*.test.js"
     ],
     "transform": {
       ".*": "<rootDir>/node_modules/babel-jest"
diff --git a/src/cons/data.js b/src/cons/data.js
index 09da092..b911c86 100644
--- a/src/cons/data.js
+++ b/src/cons/data.js
@@ -1,20 +1,20 @@
-const obj = (name, value) => {
-  const o = {};
-  o[name] = value;
-  return o;
-};
+// dataFromFiles reads the contents of each file given and returns a
+// Map of the filenames to file contents.
+async function dataFromFiles(readEncoded, files) {
+  const result = new Map();
+  await Promise.all(files.map(f => readEncoded(f).then(c => {
+    result.set(f, c);
+  })));
+  return result;
+}
 
-// Given `dir` and `read` procedures, construct a map of filename to
-// file contents (as strings).
+// Given `dir` and `read` procedures, construct a map of file basename
+// to file contents (as strings).
 const dataFromDir = ({ dir, read, Encoding }) => async function data(path) {
   const d = dir(path);
-  const files = d.files.filter(({ isdir }) => !isdir);
-  const data0 = files.map(
-    ({ name }) => read(`${path}/${name}`, { encoding: Encoding.String })
-      .then(str => obj(name, str)),
-  );
-  const data1 = await Promise.all(data0);
-  return data1.reduce((a, b) => Object.assign(a, b), {});
+  const files = d.files.filter(({ isdir }) => !isdir).map(({ name }) => name);
+  const readFile = f => read(`${path}/${f}`, { encoding: Encoding.String });
+  return dataFromFiles(readFile, files);
 };
 
-export default dataFromDir;
+export { dataFromFiles, dataFromDir };
diff --git a/src/cons/overlay.js b/src/cons/overlay.js
new file mode 100644
index 0000000..8621fc5
--- /dev/null
+++ b/src/cons/overlay.js
@@ -0,0 +1,57 @@
+// This module provides procedures for applying Kustomize-like
+// [overlays](https://github.com/kubernetes-sigs/kustomize/blob/master/docs/kustomization.yaml).
+
+// In Kustomize, a configuration is given in a `kustomize.yaml` file;
+// here we'll interpret an object (which can of course be loaded from
+// a file). In a `kustomize.yaml` you refer to files from which to
+// load or generate resource manifests, and transformations to apply
+// to all or some resources.
+//
+// The mechanism for composing configurations is to name `bases` in
+// the `kustomize.yaml` file; these are evaluated and included in the
+// resources.
+//
+//     Promise [Resource]
+//
+// There are different kinds of transformations, but they amount to
+//
+//     Resource -> Resource
+//
+// The approach taken here is
+//   0. load the file
+//   1. assemble all the transformations mentioned in various ways in the kustomize object;
+//   2. assemble all the resources, mentioned in various ways, in the kustomize object;
+//   2. run each resource through the transformations.
+//
+// Easy peasy!
+
+// overlay constructs an interpreter which takes an overlay object (as
+// would be parsed from a `kustomize.yaml`) and constructs a set of
+// resources to write out.
+const overlay = ({ read, Encoding }) => async function assemble(path, config) {
+  const readObj = f => read(`${path}/${f}`, { encoding: Encoding.JSON });
+  const {
+    resources: resourceFiles = [],
+    bases: baseFiles = [],
+    patches: patchFiles = [],
+  } = config;
+
+  const transforms = [];
+  patchFiles.forEach(f => {
+    transforms.append(readObj(f).then(interpretPatch));
+  });
+
+  // TODO: add the other kinds of transformation: imageTags,
+  // globalAnnotations, etc.
+
+  let resources = [];
+  baseFiles.forEach(f => {
+    const obj = readObj(`${path}/${f}/kustomize.yaml`);
+    resources = resources.concat(obj.then(o => assemble(`${path}/${f}`, o)));
+  });
+
+  resourceFiles.forEach(f => resources.append(readObj(f)));
+  return await Promise.all(resources);
+}
+
+export default overlay;
diff --git a/tests/data.test.js b/tests/data.test.js
index 7005e8a..48086be 100644
--- a/tests/data.test.js
+++ b/tests/data.test.js
@@ -1,19 +1,4 @@
-import dataFromDir from '../src/cons/data';
-
-function dir(path) {
-  if (path !== 'config') {
-    throw new Error(`asked for dir of ${path}`);
-  }
-  return {
-    files: [
-      {name: 'foo.yaml', isdir: false},
-      {name: 'bar.yaml', isdir: false},
-      {name: 'baz', isdir: true}
-    ]
-  };
-}
-
-const Encoding = { 'String': "string" };
+import { dataFromFiles, dataFromDir } from '../src/cons/data';
 
 const foo = `---
 conf:
@@ -28,25 +13,38 @@ stuff:
   - 3
 `;
 
-async function read(path, { encoding }) {
-  if (encoding != 'string') {
-    throw new Error(`asked for wrong encoding ${encoding}`);
-  }
-  switch (path) {
-  case 'config/foo.yaml':
-    return foo;
-  case 'config/bar.yaml':
-    return bar;
-  default:
-    throw new Error(`asked for not-a-file ${path}`);
-  }
-}
+import { fs, Encoding } from './mock';
+
+const { dir, read } = fs({
+  'config': {
+    files: [
+      {name: 'foo.yaml', isdir: false},
+      {name: 'bar.yaml', isdir: false},
+      {name: 'baz', isdir: true}
+    ]
+  },
+}, {
+  'config/foo.yaml': { string: foo },
+  'config/bar.yaml': { string: bar },
+});
+
+test('data from files', () => {
+  const files = ['config/foo.yaml', 'config/bar.yaml']
+  const readFile = f => read(f, { encoding: Encoding.String });
+  expect.assertions(1);
+  dataFromFiles(readFile, files).then(v => {
+    expect(v).toEqual(new Map([
+      ['config/foo.yaml', foo],
+      ['config/bar.yaml', bar],
+    ]));
+  });
+});
 
 test('generate data from dir', () => {
   expect.assertions(1);
   const data = dataFromDir({ dir, read, Encoding });
-  return data('config').then(d => expect(d).toEqual({
-    'foo.yaml': foo,
-    'bar.yaml': bar
-  }));
+  return data('config').then(d => expect(d).toEqual(new Map([
+    ['foo.yaml', foo],
+    ['bar.yaml', bar],
+  ])));
 });
diff --git a/tests/mock.js b/tests/mock.js
new file mode 100644
index 0000000..51fae14
--- /dev/null
+++ b/tests/mock.js
@@ -0,0 +1,35 @@
+const dir = dirs => path => {
+  if (path in dirs) {
+    return dirs[path];
+  }
+  throw new Error(`path not found ${path}`);
+};
+
+const read = files => async function r(path, { encoding }) {
+  if (path in files) {
+    const encodings = files[path];
+    if (encoding in encodings) {
+      return encodings[encoding];
+    }
+    throw new Error(`no value for encoding "${encoding}"`);
+  }
+  throw new Error(`file not found ${path}`);
+};
+
+function fs(dirs, files) {
+  return {
+    dir: dir(dirs),
+    read: read(files),
+    Encoding: Encoding,
+  };
+}
+
+// It's not important what the values are, just that we use them
+// consistently.
+const Encoding = Object.freeze({
+  String: 'string',
+  JSON: 'json',
+  Bytes: 'bytes',
+});
+
+export { fs, Encoding };
diff --git a/tests/overlay.test.js b/tests/overlay.test.js
new file mode 100644
index 0000000..3e93ff9
--- /dev/null
+++ b/tests/overlay.test.js
@@ -0,0 +1,11 @@
+import overlay from '../src/cons/overlay';
+import { fs, Encoding } from './mock';
+
+test('trivial overlay: no bases, resources, patches', () => {
+  const o = overlay({});
+  const { dir, read } = fs({}, {});
+  expect.assertions(1);
+  o('config', {dir, read }).then(v => {
+    expect(v).toEqual([]);
+  });
+});