static <A> Chain<A> chainPlus(Chain<A> chain, A a) {
  ret Chain<A>(a, chain);
}

static <A> ReverseChain<A> chainPlus(ReverseChain<A> chain, A a) {
  ret ReverseChain<A>(chain, a);
}

ifclass AppendableChain
static <A> Chain<A> chainPlus(AppendableChain<A> chain, A a) {
  ret AppendableChain<A>(a, chain);
}
endif