  It has been a fascinating topic in the study of boundary layer theory about
the well-posedness of Prandtl equation that was derived in 1904. Recently, new
ideas about cancellation to overcome the loss of tangential derivatives were
obtained so that Prandtl equation can be shown to be well-posed in Sobolev
spaces to avoid the use of Crocco transformation as in the classical work of
Oleinik. This short note aims to show that the cancellation mechanism is in
fact related to some intrinsic directional derivative that can be used to
recover the tangential derivative under some structural assumption on the fluid
near the boundary.
