f(x, y) = x^(4) + y^(4) - (4 * (x * y)) + 1
a = -1 (slider from -2 to 2, step 0.25)
p: x = a
r1 = IntersectPath(p, f)
b = -1.75 (slider from -2 to 2, step 0.25)
q: y = b
r2 = IntersectPath(q, f)
px(x, y) = (4 * x^(3)) - (4 * y)
py(x, y) = (4 * y^(3)) - (4 * x)
c: px(a, b)
A: (a, b, f(a, b))
d: py(a, b)