We present a new synchronous programming language named \Name{SL} based on \Name{Esterel}, in which hypothesis about signal presences or absences are not allowed. Thus, one can decide that a signal was absent during one instant only at the end of this instant, and so reaction to this absence is delayed. \Name{Esterel} «causality problems» are avoided at the price of replacing strong preemptions by weak ones. An operational semantics based on rewriting rules is given and an implementation is described which allows either to directly execute programs, or to produce automata.