+Returns the maximum signal value for which a corresponding symbolic
+name or message is available. Note that in the case where we use the
+@code{sys_siglist} supplied by the system, it is possible for there to
+be more symbolic names than messages, or vice versa. In fact, the
+manual page for @code{psignal(3b)} explicitly warns that one should
+check the size of the table (@code{NSIG}) before indexing it, since
+new signal codes may be added to the system before they are added to
+the table. Thus @code{NSIG} might be smaller than value implied by
+the largest signo value defined in @code{<signal.h>}.