As indicated by these warnings generated in a clean build:

File ".../src/elpi/translation.elpi", line 280, characters 0-128:
Warning: XTy is linear: name it _XTy (discard) or XTy_ (fresh variable)
[elpi.typecheck,elpi,default]
File ".../src/elpi/translation.elpi", line 280, characters 0-128:
Warning: X is linear: name it _X (discard) or X_ (fresh variable)