internal traceback into a list
        of the pyparsing expressions that caused the exception to be raised.

        Parameters:

        - depth (default=16) - number of levels back in the stack trace to list expression
          and function names; if None, the full stack trace names will be listed; if 0, only
          the failing input line, marker, and exception string will be shown

        Returns a multi-line string listing the ParserElements and/or function names in the
        exception's stack trace.

        Example::

            expr = pp.Word(pp.nums) * 3
            try:
                expr.parse_string("123 456 A789")
            except pp.ParseException as pe:
                print(pe.explain(depth=0))

        prints::

            123 456 A789
                    ^
            ParseException: Expected W:(0-9), found 'A'  (at char 8), (line:1, col:9)

        Note: the diagnostic output will include string representations of the expressions
        that failed to parse. These representations will be more helpful if you use `set_name` to
        give identifiable names to your expressions. Otherwise they will use the default string
        forms, which may be cryptic to read.

        Note: pyparsing's default truncation of exception tracebacks may also truncate the
        stack of expressions that are displayed in the ``explain`` output. To get the full listing
        of parser expressions, you may have to set ``ParserElement.verbose_stacktrace = True``
        )