Thanks a lot for example,
I think I understand it better now
If variables ##x, y, z## were independent then ## \left( \frac {\partial w} {\partial x} \right)_y \equiv \left( \frac {\partial w} {\partial x} \right) _z##
but if ##x, y, z## are not independent, then
## \left( \frac {\partial w}...