Modified eqvt_tac to avoid failure due to introduction rules
whose premises contain variables that do not occur in the
conclusion.
module Codegen where {
import qualified Nat;
class Null a where {
nulla :: a;
};
heada :: (Codegen.Null a) => [a] -> a;
heada (x : xs) = x;
heada [] = Codegen.nulla;
null_option :: Maybe a;
null_option = Nothing;
instance Codegen.Null (Maybe b) where {
nulla = Codegen.null_option;
};
dummy :: Maybe Nat.Nat;
dummy = Codegen.heada [Just (Nat.Suc Nat.Zero_nat), Nothing];
}