fix: preserve stack until operation completes

src/rpn-calculator.js

@@ -369,9 +369,13 @@ class RpnCalculator {
       throw new Error('Stack underflow');
     }
 
-    const operands = argCount > 0 ? this.stack.splice(0, argCount).reverse() : [];
+    const operands = argCount > 0 ? this.stack.slice(0, argCount).reverse() : [];
     const result = operation.execute(this, ...operands, ...args);
 
+    if (argCount > 0) {
+      this.stack.splice(0, argCount);
+    }
+
     if (Array.isArray(result)) {
       for (let i = result.length - 1; i >= 0; i -= 1) {
         this.push(result[i]);