toFunType (FApp c [_,_,_,ity])
    | c == sUN "C_Fn" = toFunType ity
toFunType (FApp c [_,_,_,ity])
    | c == sUN "C_Fn" = toFunType ity
toFunType (FApp c [_,_,_,ity])
    | c == sUN "C_Fn" = toFunType ity