trap 'echo USR1 $BASHPID' USR1
trap 'echo foo $BASH_SUBSHELL' 0

trap | cat
echo between
( trap )
echo between 2
{ trap; } | cat

( echo in subshell: $BASH_SUBSHELL )
( echo in subshell pipeline: $BASH_SUBSHELL ) | cat

{ echo group pipeline: $BASH_SUBSHELL;
  trap 'echo EXIT-group.$BASH_SUBSHELL' EXIT; } | cat