  The superposition principle is fundamental to quantum theory. Yet a recent
no-go theorem has proved that quantum theory forbids superposition of unknown
quantum states, even with nonzero probability. The implications of this result,
however, remain poorly understood so far. In this paper we show that the
existence of a protocol that superposes two unknown pure states with nonzero
probability (allowed to vary over input states) leads to violation of other
no-go theorems. In particular, such a protocol can be used to perform certain
state discrimination and cloning tasks that are forbidden not only in quantum
theory but in no-signaling theories as well.
