Imagine that there is nature coordinate system on a spacial curve line. {\bf
t} is unit vector on the tangent, {\bf n} is unit vector on the normal line.
{\bf b}
is unit vector on the subsidiary normal line. These three vectors are
perpendicular to each other.
If vector {\bf b} is changed into {\bf b'}. Vector {\bf n} and {\bf b'} form
into a polar coordinate system. These two and {\bf t} will form into a new
spacial coordiante system. I wonder is this coordiante system posssible.
Why I make this change? Because nature coordinate system can not describe
the