initTc: unsolved constraints
ghc: panic! (the 'impossible' happened)
(GHC version 8.0.2 for x86_64-unknown-linux):
initTc: unsolved constraints
WC {wc_insol =
[W] ltail_ae2e :: t_ae2d[tau:1] (CHoleCan: ltail)
[W] lhead_ae2h :: t_ae2g[tau:1] (CHoleCan: lhead)}