?? "derivation at a pair (p,q) of points" ... generalizing "derivation at a point p" ...

d(fg) = d(f)g(p)+f(p)d(g)

d(fg) = d(f)g(p)+f(q)d(g) ??? ... ??or something ?? ...

??for example d(f) := f(q)-f(p) ??...

d(fg) = f(q)g(q)-f(p)g(p) =?= (f(q)-f(p))g(p)+f(q)(g(q)-g(p)) = d(f)g(p)+f(q)d(g)

??seems correct ??? ...