-- How to shrink Commands

shrinker :: Model Symbolic -> Command Symbolic -> [Command Symbolic]
shrinker _ (NewBook tag key x y) = [ NewBook tag key x y' | y' <- shrink y ] ++
                                   [ NewBook tag key x' y | x' <- shrink x ]
shrinker :: Model Symbolic -> Command Symbolic -> [Command Symbolic]
shrinker _ (NewBook tag key x y) = [ NewBook tag key x y' | y' <- shrink y ] ++
                                   [ NewBook tag key x' y | x' <- shrink x ]
shrinker _ (NewBook tag key x y) = [ NewBook tag key x y' | y' <- shrink y ] ++
                                   [ NewBook tag key x' y | x' <- shrink x ]