This enclosure leaves the corner quite open.
White may be able to play at (as slide or submarine). Then this sequence is standard,
being correct shape (at a leaves bad aji).
There are various ways to enclose the corner more securely: was an idea of Kubouchi Shuchi. Black a is common but Black b is probably most popular from the point of view of corner safety.
3554 enclosure opening
This isn't often seen in pro games. White a, Black b is a continuation seen in a couple of examples.