in documentation of "signal"
http://docs.python.org/library/signal.html
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signal.SIG_DFL
This is one of two standard signal handling options; it will simply 
perform the default function for the signal. For example, on most 
systems the default action for SIGQUIT is to dump core and exit, while 
the default action for SIGCLD is to simply ignore it.
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Typo:
SIGCLD
should be
SIGCHLD