base.Map[K,V]

imm .get(k: K): mut base.Opt[imm V] -> …,
read .keyEq(k1: K, k2: K): imm base.Bool,
read .isEmpty: imm base.Bool -> …,
imm .get(k: K): mut base.Opt[imm V] -> …,
read .keyEq(k1: K, k2: K): imm base.Bool,
read .isEmpty: imm base.Bool -> …,