On: splice() and the ghost of set_fs()

[shaded-enmity]

You still need to implement a CI rule so that I don't just call `set_fs()` without using `preferred_syntax(set_fs)`, don't you?

[viraptor → shaded-enmity]

We're talking a theoretical language here, so... maybe? It could be also that set_fs is usable only in the preferred_syntax mode.