If purely for getting it done then sure, but eval is slow. From what I understand it can't be optimized by the compiler because it's all runtime, so it's insecure and slow.

Am consistently getting:

Appending (b a) to (c a):
Test Passed
Test Passed
Test Passed
Test Failed: Append leaves argument untouched.

and my test units are returning

l1 // (2 1)
l2 // (Sauce b a)
l1.append(l2) // (2 1 Sauce b a)
------- arguments after append ---------
l1 // still (2 1)
l2 // still (Sauce b a)

Neither l1 nor l2 are getting modified so the error is making no sense.

Expanded the description and test cases. The desired result is to not get a throw error.