- entering the debugger:

  - var. debug-on-error, fcn. toggle-debug-on-error

  - var. debug-on-signal, fcn. toggle-debug-on-signal  debugs even
    handled errors

  - (debug-on-entry FCN) (FCN -- symbol)
    M-x debug-on-entry
    (cancel-debug-on-entry FCN)
    M-x cancel-debug-on-entry

  - explicit entry: call (debug) anywhere inside your prog
    (preferably at a place where the return value is ignored


- inside *Backtrace* buffer:

  - most important cmds: "d" (step into), "c" (step over), "e" (eval
    expression) (TODO: verify)