Function: trap
Section: programming
C-Name: trap0
Prototype: D"",r,DIDI
Help: trap({err}, {rec}, {seq}): try to execute seq, trapping error err (all
 of them if err ommitted); sequence rec is executed if the error occurs and
 is the result of the command. When seq is omitted, define rec as a default
 handler for error err (a break loop will be started if rec omitted). If rec
 is the empty string "" pop out the last default handler

