function do_debug(){
  set +x
  trap - DEBUG
  unset PS4
  while true; do
    local _debug_cmd=""
    read -p '> ' _debug_cmd
    test -z "$_debug_cmd" && break
    eval $_debug_cmd
  done
  export PS4='+(${BASH_SOURCE}:${LINENO}): ${FUNCNAME[0]:+${FUNCNAME[0]}(): }'
  trap "do_debug" DEBUG
  set -x
}

function debug(){
  #DOC:  start a step debugger
  set -x
  export PS4='+(${BASH_SOURCE}:${LINENO}): ${FUNCNAME[0]:+${FUNCNAME[0]}(): }'
  trap "do_debug" DEBUG
}