Return an iterator providing modifiable access to the first (i.e., ordered least) value_type object in this map whose key is greater-than or equal-to the specified key, and the past-the-end iterator if this map does not contain a value_type object whose key is greater-than or equal-to key. Note that this function returns the first position before which a value_type object having an equivalent key could be inserted into the ordered sequence maintained by this map, while preserving its ordering.