If I type
I get the error - "Generic typesetting error; LaTeX failed. Please report a bug."
Use a period '.' to end that line, so
(I really should fix this; it essentially cannot know how to print \del until it has processed this line, but things get executed in opposite order).
Track this at https://github.com/kpeeters/cadabra2/issues/84.
Now fixed on github.