Create a Muriel-like language and implementation
Python 3 and &quot;The minimum TC subset of Underload but with the characters changed to 2019, reversed, and with a 0 on top&quot;, 156 129 bytes

c,n,*s=input(),0,[]
while c:
 i,*c=c;q=&quot;6&quot;&gt;i
 if n:n+=q*int(i)-q;s[n and-1]+=i,
 elif q:n=&quot;1&quot;&lt;i;s+=s[-1]*(1-n),
 else:c=s.pop()+c

Try it online!
Explanations in the header, ungolfed code in the footer.