Proof: Everywhere Tangent to Curve??
If the function v depends on x and y, v(x,y) and we know there exists some function psi(x,y) such that
v[SIZE="1"]x = partial w.r.t (y) of psi
v[SIZE="1"]y= -(partial w.r.t (x) of psi)
show that the curves psi(x,y) = constant, are everywhere tangent to v.