def caller_trace(back=0):
    """
    Trace caller stack and save info into global dicts, which
    are printed automatically at the end of SCons execution.
    """
    global caller_bases, caller_dicts
    import traceback
    tb = traceback.extract_stack(limit=3+back)
    tb.reverse()
    callee = tb[1][:3]
    caller_bases[callee] = caller_bases.get(callee, 0) + 1
    for caller in tb[2:]:
        caller = callee + caller[:3]
        try:
            entry = caller_dicts[callee]
        except KeyError:
            caller_dicts[callee] = entry = {}
        entry[caller] = entry.get(caller, 0) + 1
        callee = caller
Trace caller stack and save info into global dicts, which
    are printed automatically at the end of SCons execution.