/* A jmpenv packages the state required to perform a proper non-local jump.
 * Note that there is a start_env initialized when perl starts, and top_env
 * points to this initially, so top_env should always be non-null.
 *
 * Existence of a non-null top_env->je_prev implies it is valid to call
 * longjmp() at that runlevel (we make sure start_env.je_prev is always
 * null to ensure this).
 *
 * je_mustcatch, when set at any runlevel to TRUE, means eval ops must
 * establish a local jmpenv to handle exception traps.  Care must be taken
 * to restore the previous value of je_mustcatch before exiting the
 * stack frame iff JMPENV_PUSH was not called in that stack frame.
 * GSAR 97-03-27
 */