These are the packages that would be merged, in order:

Calculating dependencies ...

!!! Problem resolving dependencies for =dev-lang/fennel-1.6.1
 done!
Dependency resolution took 0.83 s (backtrack: 0/20).


!!! The ebuild selected to satisfy "=dev-lang/fennel-1.6.1" has unmet requirements.
- dev-lang/fennel-1.6.1::gentoo USE="" LUA_SINGLE_TARGET="-lua5-3 -lua5-4 -luajit"

  The following REQUIRED_USE flag constraints are unsatisfied:
    exactly-one-of ( lua_single_target_luajit lua_single_target_lua5-3 lua_single_target_lua5-4 )