def read_at(self, d, index=False):
for i, iv in enumerate(self):<EOL><INDENT>if iv.spans(d):<EOL><INDENT>return i if index else iv<EOL><DEDENT><DEDENT>return None<EOL>
Get the index of the interval at a particular 'depth' (though this
    might be an elevation or age or anything).

Args:
    d (Number): The 'depth' to query.
    index (bool): Whether to return the index instead of the interval.

Returns:
    Interval: The interval, or if ``index==True`` the index of the
        interval, at the specified 'depth', or ``None`` if the depth is
        outside the striplog's range.