155 bytes :D

155 bytes :D is is an esoteric programming language designed when User:Xyzzy tried to prove repeated f-string formatting codes Turing-Complete. He has not succeeded in achieving said goal.

The language is, in fact, Turing-Complete.

The exact mechanics of how this language works is left as an exercise to the reader.

Examples

4-state Busy beaver

0ab 1ac _ad 0bb0 abe bbf cbg dbh 1bc0 _bd0 #bq0 0cb1 1cc1 aci bcj cck dcl _cd1 #cq1 0db_ 1dc_ _dd_ adm bdn cdo ddp 0eab01 1eab11 _ea_0a0 0fa01a 1fa11a _fa_0b0 0ga0#1 1ga1#1 _ga_0c0 0had01 1had11 _ha_0d0 0ia01b 1ia11b _ia_0a1 0ja00c 1ja10c _ja_0b1 0ka01d 1ka11d _ka_0c1 0laa00 1laa10 _la_0d1 0ma0a0_ 1ma1a0_ _ma_0a0_ 0na0b0_ 1na1b0_ _na_0b0_ 0oa0c0_ 1oa1c0_ _oa_0c0_ 0pa0d0_ 1pa1d0_ _pa_0d0_ 0qq0 1qq1 _qr _rs

Implementation

Python:

p,t,code=dict(),"_a_a",input()
for s in code.split():p[s[:2]]=s[2:]
try:
 while True:t=f"{(u:=p[t[-2:]])[1:]}{t:.{len(t)-2}}{u:.1}"
except:print(t[-2::-1]) ]