A signal is a value that carries no information.
It typically indicates completion of a call, when there is no other meaningful value to return.
It is analogous to
void in Java or
() in ML.
Notable signal operations include:
Orc signals don't correspond to any Java value, so if a signal
is passed to Java code, it will be as an
java.lang.Object of a type not specified here.
A return of type
void from Java code is converted into a signal return value.