diff --git a/src/lib/_lexer.ts b/src/lib/_lexer.ts
index 8445605..d6a4474 100644
--- a/src/lib/_lexer.ts
+++ b/src/lib/_lexer.ts
@@ -8,7 +8,7 @@ import { debug } from './logger';
 // These are the regex patterns for parsing. They must not have any
 // capturing groups. They are used during lexing and will be
 // checked by name during parsing.
-const PATTERNS = {
+const PATTERNS: { [key: string]: string } = {
   COMMENT: '#.*$',
   STRING: '"[^"\\\\]*(?:\\\\.[^"\\\\]*)*"',
   ALIGNMENT: '~(?:[a-z].?)?[0-9]+(?:,[0-9]+)*',
diff --git a/src/lib/model.ts b/src/lib/model.ts
index 32dedf4..a02d903 100644
--- a/src/lib/model.ts
+++ b/src/lib/model.ts
@@ -81,10 +81,10 @@ export class Model {
     const deifs: Map<Constant, Array<_Dereified>> = new Map();
     if (reifications) {
       for (const [role, concept, source, target] of reifications) {
-        if (reifs[role] === undefined) {
-          reifs[role] = [];
+        if (!reifs.has(role)) {
+          reifs.set(role, []);
         }
-        reifs[role].push([concept, source, target]);
+        reifs.get(role)!.push([concept, source, target]);
         if (!deifs.has(concept)) {
           deifs.set(concept, []);
         }
@@ -258,7 +258,7 @@ export class Model {
    * Return `true` if `role` can be reified.
    */
   isRoleReifiable(role: Role): boolean {
-    return role in this.reifications;
+    return this.reifications.has(role);
   }
 
   /**
@@ -285,10 +285,10 @@ export class Model {
   reify(triple: Triple, options: ModelReifyOptions = {}): _Reification {
     const { variables } = options;
     const [source, role, target] = triple;
-    if (!(role in this.reifications)) {
+    if (!this.reifications.has(role)) {
       throw new ModelError(`'${role}' cannot be reified`);
     }
-    const [concept, sourceRole, targetRole] = this.reifications[role][0];
+    const [concept, sourceRole, targetRole] = this.reifications.get(role)![0];
 
     let variable = '_';
     if (variables) {
diff --git a/src/lib/transform.spec.ts b/src/lib/transform.spec.ts
index be68497..766e4a9 100644
--- a/src/lib/transform.spec.ts
+++ b/src/lib/transform.spec.ts
@@ -61,7 +61,7 @@ test('canonicalize_roles_amr_codec', (t) => {
 test('reify_edges_default_codec', (t) => {
   const decode = defCodec.decode.bind(defCodec);
   const norm = makeNorm(reifyEdges, defModel);
-  const form = makeForm((g: Graph, { indent }: { indent?: number }) =>
+  const form = makeForm((g: Graph, { indent }: { indent?: number | null }) =>
     defCodec.encode(g, { indent }),
   );
 
@@ -75,7 +75,7 @@ test('reify_edges_default_codec', (t) => {
 test('reify_edges_amr_codec', (t) => {
   const decode = amrCodec.decode.bind(amrCodec);
   const norm = makeNorm(reifyEdges, amrModel);
-  const form = makeForm((g: Graph, { indent }: { indent?: number }) =>
+  const form = makeForm((g: Graph, { indent }: { indent?: number | null }) =>
     amrCodec.encode(g, { indent }),
   );
 
@@ -101,7 +101,7 @@ test('reify_edges_amr_codec', (t) => {
 test('dereify_edges_default_codec', (t) => {
   const decode = defCodec.decode.bind(defCodec);
   const norm = makeNorm(dereifyEdges, defModel);
-  const form = makeForm((g: Graph, { indent }: { indent?: number }) =>
+  const form = makeForm((g: Graph, { indent }: { indent?: number | null }) =>
     defCodec.encode(g, { indent }),
   );
 
@@ -125,7 +125,7 @@ test('dereify_edges_default_codec', (t) => {
 test('dereify_edges_amr_codec', (t) => {
   const decode = amrCodec.decode.bind(amrCodec);
   const norm = makeNorm(dereifyEdges, amrModel);
-  const form = makeForm((g: Graph, { indent }: { indent?: number }) =>
+  const form = makeForm((g: Graph, { indent }: { indent?: number | null }) =>
     amrCodec.encode(g, { indent }),
   );
 
@@ -148,7 +148,7 @@ test('dereify_edges_amr_codec', (t) => {
 test('reify_attributes', (t) => {
   const decode = defCodec.decode.bind(defCodec);
   const norm = reifyAttributes;
-  const form = makeForm((g: Graph, { indent }: { indent?: number }) =>
+  const form = makeForm((g: Graph, { indent }: { indent?: number | null }) =>
     defCodec.encode(g, { indent }),
   );
 
@@ -165,7 +165,7 @@ test('indicate_branches', (t) => {
     (x, { model }: { model: Model }) => indicateBranches(x, model),
     defModel,
   );
-  const form = makeForm((g: Graph, { indent }: { indent?: number }) =>
+  const form = makeForm((g: Graph, { indent }: { indent?: number | null }) =>
     defCodec.encode(g, { indent }),
   );
 
diff --git a/src/lib/tree.spec.ts b/src/lib/tree.spec.ts
index 9763119..3c55b52 100644
--- a/src/lib/tree.spec.ts
+++ b/src/lib/tree.spec.ts
@@ -214,7 +214,7 @@ test('walk', (t) => {
 //         assert _vars(t) == ['a', 'b']
 
 test('reset_variables', (t) => {
-  const _vars = (t) => t.nodes().map(([v]) => v);
+  const _vars = (t: Tree) => t.nodes().map(([v]) => v);
 
   const t1 = new Tree(one_arg_node());
   t.deepEqual(_vars(t1), ['a', 'b']);
diff --git a/src/lib/tree.ts b/src/lib/tree.ts
index 8383c91..5086bc5 100644
--- a/src/lib/tree.ts
+++ b/src/lib/tree.ts
@@ -168,7 +168,7 @@ const _nodes = (node: Node): Node[] => {
   return ns;
 };
 
-function* _walk(node: Node, path: number[]) {
+function* _walk(node: Node, path: number[]): Generator<_Step> {
   const branches = node[1] ?? [];
   for (const [i, branch] of branches.entries()) {
     const curpath = path.concat([i]);
diff --git a/tsconfig.json b/tsconfig.json
index 5d12667..a102e61 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -10,16 +10,7 @@
     "inlineSourceMap": true,
     "esModuleInterop": true /* Enables emit interoperability between CommonJS and ES Modules via creation of namespace objects for all imports. Implies 'allowSyntheticDefaultImports'. */,
     "resolveJsonModule": true /* Include modules imported with .json extension. */,
-
-    // "strict": true /* Enable all strict type-checking options. */,
-
-    /* Strict Type-Checking Options */
-    // "noImplicitAny": true /* Raise error on expressions and declarations with an implied 'any' type. */,
-    "strictNullChecks": true /* Enable strict null checks. */,
-    // "strictFunctionTypes": true /* Enable strict checking of function types. */,
-    // "strictPropertyInitialization": true /* Enable strict checking of property initialization in classes. */,
-    // "noImplicitThis": true /* Raise error on 'this' expressions with an implied 'any' type. */,
-    // "alwaysStrict": true /* Parse in strict mode and emit "use strict" for each source file. */,
+    "strict": true /* Enable all strict type-checking options. */,
 
     /* Additional Checks */
     "noUnusedLocals": true /* Report errors on unused locals. */,