diff --git a/src/vm/vm.ts b/src/vm/vm.ts
index 532acec..2521a38 100644
--- a/src/vm/vm.ts
+++ b/src/vm/vm.ts
@@ -53,6 +53,12 @@ export class Scope {
     makeChild(): Scope {
         return new Scope(this)
     }
+
+    clone(): Scope {
+        const s = new Scope
+        s.entries = {...this.entries}
+        return s
+    }
 }
 
 /**
@@ -81,11 +87,31 @@ export type TermOperator = {
 }
 
 export class ExecutionContext {
+    private history: [StrRVal, Scope][] = []
+
     constructor(
         private subject: StrRVal,
         private scope: Scope,
     ) {}
 
+    public pushHistory() {
+        this.history.push([this.subject, this.scope.clone()])
+    }
+
+    public restoreHistory() {
+        const [subject, scope] = this.popHistory()
+        this.subject = subject
+        this.scope = scope
+    }
+
+    public popHistory(): [StrRVal, Scope] {
+        const state = this.history.pop()
+        if ( !state ) {
+            throw new Error('No history to undo')
+        }
+        return state
+    }
+
     async replaceSubject(operator: (sub: StrRVal) => Awaitable<StrRVal>) {
         this.subject = await operator(this.subject)
     }
@@ -201,7 +227,14 @@ export class StrVM {
     ) {}
 
     public async runInPlace<TReturn>(operator: (ctx: ExecutionContext) => Awaitable<TReturn>): Promise<TReturn> {
-        return operator(this.context)
+        this.context.pushHistory()
+        try {
+            return await operator(this.context)
+        } catch (e: unknown) {
+            // If we got an error, return to the previous state:
+            this.context.restoreHistory()
+            throw e
+        }
     }
 
     public async tapInPlace(operator: (ctx: ExecutionContext) => Awaitable<unknown>): Promise<this> {