min.:   J = int_0^1 (u-x)**2 dt

  sub to: x' = u;  x(0) = 1
  The minimum principle gives the bvp problem:

  x' = u; p' = 2*(u-x); with x(0) = 1, p(1) = 0

  where u = x - p/2.
  Note: lambda is renamed as p!

u =  y[0] - y[1]/2.

return [u, 2*(u-y[0])]

return [ya[0]-1, yb[1]]