add union,unions, and sempty to SetLike class, remove 'Monoid' superclass
add union,unions, and sempty to SetLike class, remove 'Monoid' superclass
shinnya/jhc-mirror,hvr/jhc,hvr/jhc,hvr/jhc,shinnya/jhc-mirror,shinnya/jhc-mirror,hvr/jhc,shinnya/jhc-mirror