diff --git a/src/runtime/define.ts b/src/runtime/define.ts
index d1a6cbe..bbc25ad 100644
--- a/src/runtime/define.ts
+++ b/src/runtime/define.ts
@@ -1,9 +1,10 @@
-import type {Variable, VariableDefinition} from "@observablehq/runtime";
+import {Runtime, type Module, type Variable, type VariableDefinition} from "@observablehq/runtime";
 import type {DisplayState} from "./display.js";
 import {clear, display, observe} from "./display.js";
-import {main} from "./index.js";
+import {library} from "./stdlib/index.js";
 import {input} from "./stdlib/generators/index.js";
 import {Mutator} from "./stdlib/mutable.js";
+import {fileAttachments} from "./stdlib/fileAttachment.js";
 
 export type DefineState = DisplayState & {
   /** the runtime variables associated with this cell */
@@ -31,61 +32,87 @@ export type Definition = {
   assets?: Map<string, string>;
 };
 
-export function define(state: DefineState, definition: Definition, observer = observe): void {
-  const {id, body, inputs = [], outputs = [], output, autodisplay, autoview, automutable} = definition;
-  const variables = state.variables;
-  const v = main.variable(observer(state, definition), {shadow: {}});
-  const vid = output ?? (outputs.length ? `cell ${id}` : null);
-  if (inputs.includes("display") || inputs.includes("view")) {
-    let displayVersion = -1; // the variable._version of currently-displayed values
-    const vd = new (v.constructor as typeof Variable)(2, v._module);
-    vd.define(
-      inputs.filter((i) => i !== "display" && i !== "view"),
-      () => {
-        const version = v._version; // capture version on input change
-        return (value: unknown) => {
-          if (version < displayVersion) throw new Error("stale display");
-          else if (state.variables[0] !== v) throw new Error("stale display");
-          else if (version > displayVersion) clear(state);
-          displayVersion = version;
-          display(state, value);
-          return value;
-        };
-      }
-    );
-    v._shadow.set("display", vd);
-    if (inputs.includes("view")) {
-      const vv = new (v.constructor as typeof Variable)(2, v._module, null, {shadow: {}});
-      vv._shadow.set("display", vd);
-      vv.define(["display"], (display) => (value: unknown) => input(display(value)));
-      v._shadow.set("view", vv);
-    }
-  } else if (!autodisplay) {
-    clear(state);
+export class NotebookInstance {
+
+  readonly runtime = Object.assign(new Runtime({ ...library, __ojs_runtime: () => this.runtime }), { fileAttachments });
+  readonly main = (this.runtime as typeof this.runtime & { main: Module }).main = this.runtime.module();
+
+  constructor() {
   }
-  variables.push(v.define(vid, inputs, body));
-  if (output != null) {
-    if (autoview) {
-      const o = unprefix(output, "viewof$");
-      variables.push(main.define(o, [output], input));
-    } else if (automutable) {
-      const o = unprefix(output, "mutable ");
-      const x = `cell ${id}`;
-      v.define(o, [x], ([mutable]) => mutable); // observe live value
-      variables.push(
-        main.define(output, inputs, body), // initial value
-        main.define(x, [output], Mutator),
-        main.define(`mutable$${o}`, [x], ([, mutator]) => mutator)
+
+  define(state: DefineState, definition: Definition, observer = observe): void {
+    const { id, body, inputs = [], outputs = [], output, autodisplay, autoview, automutable } = definition;
+    const variables = state.variables;
+    const v = this.main.variable(observer(state, definition), { shadow: {} });
+    const vid = output ?? (outputs.length ? `cell ${id}` : null);
+    if (inputs.includes("display") || inputs.includes("view")) {
+      let displayVersion = -1; // the variable._version of currently-displayed values
+      const vd = new (v.constructor as typeof Variable)(2, v._module);
+      vd.define(
+        inputs.filter((i) => i !== "display" && i !== "view"),
+        () => {
+          const version = v._version; // capture version on input change
+          return (value: unknown) => {
+            if (version < displayVersion) throw new Error("stale display");
+            else if (state.variables[0] !== v) throw new Error("stale display");
+            else if (version > displayVersion) clear(state);
+            displayVersion = version;
+            display(state, value);
+            return value;
+          };
+        }
       );
+      v._shadow.set("display", vd);
+      if (inputs.includes("view")) {
+        const vv = new (v.constructor as typeof Variable)(2, v._module, null, { shadow: {} });
+        vv._shadow.set("display", vd);
+        vv.define(["display"], (display) => (value: unknown) => input(display(value)));
+        v._shadow.set("view", vv);
+      }
+    } else if (!autodisplay) {
+      clear(state);
     }
-  } else {
-    for (const o of outputs) {
-      variables.push(main.variable(true).define(o, [vid!], (exports) => exports[o]));
+    variables.push(v.define(vid, inputs, body));
+    if (output != null) {
+      if (autoview) {
+        const o = this.unprefix(output, "viewof$");
+        variables.push(this.main.define(o, [output], input));
+      } else if (automutable) {
+        const o = this.unprefix(output, "mutable ");
+        const x = `cell ${id}`;
+        v.define(o, [x], ([mutable]) => mutable); // observe live value
+        variables.push(
+          this.main.define(output, inputs, body), // initial value
+          this.main.define(x, [output], Mutator),
+          this.main.define(`mutable$${o}`, [x], ([, mutator]) => mutator)
+        );
+      }
+    } else {
+      for (const o of outputs) {
+        variables.push(this.main.variable(true).define(o, [vid!], (exports) => exports[o]));
+      }
     }
   }
-}
 
-function unprefix(name: string, prefix: string): string {
-  if (!name.startsWith(prefix)) throw new Error(`expected ${prefix}: ${name}`);
-  return name.slice(prefix.length);
+  protected unprefix(name: string, prefix: string): string {
+    if (!name.startsWith(prefix)) throw new Error(`expected ${prefix}: ${name}`);
+    return name.slice(prefix.length);
+  }
 }
+
+const defaultInstance = new NotebookInstance();
+
+export const runtime = defaultInstance.runtime;
+export const main = defaultInstance.main;
+
+main.constructor.prototype.defines = function (this: Module, name: string): boolean {
+  return (
+    this._scope.has(name) ||
+    this._builtins.has(name) ||
+    this._runtime._builtin._scope.has(name)
+  );
+};
+
+export function define(state: DefineState, definition: Definition, observer = observe): void {
+  defaultInstance.define(state, definition, observer);
+}
\ No newline at end of file
diff --git a/src/runtime/index.ts b/src/runtime/index.ts
index d1594ac..6b5a5fa 100644
--- a/src/runtime/index.ts
+++ b/src/runtime/index.ts
@@ -1,8 +1,3 @@
-import type {Module} from "@observablehq/runtime";
-import {Runtime} from "@observablehq/runtime";
-import {fileAttachments} from "./stdlib/fileAttachment.js";
-import {library} from "./stdlib/index.js";
-
 export * from "./define.js";
 export * from "./display.js";
 export * from "./inspect.js";
@@ -12,14 +7,3 @@ export type * from "./stdlib/databaseClient.js";
 export type * from "./stdlib/fileAttachment.js";
 export {DatabaseClient} from "./stdlib/databaseClient.js";
 export {FileAttachment, registerFile} from "./stdlib/fileAttachment.js";
-
-export const runtime = Object.assign(new Runtime({...library, __ojs_runtime: () => runtime}), {fileAttachments});
-export const main = (runtime as typeof runtime & {main: Module}).main = runtime.module();
-
-main.constructor.prototype.defines = function (this: Module, name: string): boolean {
-  return (
-    this._scope.has(name) ||
-    this._builtins.has(name) ||
-    this._runtime._builtin._scope.has(name)
-  );
-};