struct jmploc

We enclose jmp_buf in a structure so that we can declare pointers to

jump locations. The global variable handler contains the location to

jump to when an exception occurs, and the global variable exception

contains a code identifying the exeception. To implement nested

exception handlers, the user should save the value of handler on entry

to an inner scope, set handler to point to a jmploc structure for the

inner scope, and restore handler on exit from the scope.