#25 abc2midi hangs/loops on this ABC example

[Gwern Branwen]

While working on ABC music generation using neural nets (https://www.gwern.net/GPT-2-music https://www.gwern.net/GPT-2-preference-learning), my rating script hung on an example. I minimized it a little bit to:

$ cat foo
X:1
L:1/4
K:C
2c/2fac'/c'/c'/c'/c'///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
$ abc2midi foo
4.24 October 13 2019 abc2midi
Warning in line-char 3-0 : No M: in header, using default
Error in line-char 4-0 : Unrecognized character: 2

This hangs for at least half an hour, without memory consumption increasing, in contrast to most ABC pieces which compile in ~1s or less.

This may be valid ABC or not, but abc2midi should terminate within a reasonable time.