Issue with the .xtc export of lambdapi files

[melanie-taprogge]

When attempting to export files with "lambdapi export --output=xtc" I am getting the error "Type symbol in a term".
This happened when attempting to export a file that combines the contents of two files (prop.lp and set.lp) from the theory U repository (https://github.com/Deducteam/lambdapi-logics/tree/master/U).

example.lp.zip