Hello

I have a problem with "Infinitesimals" for the system of equations. 

"Infinitesimals" Counts the system for (x, t) but shows an empty string for (x, y, z, t).

When I add to the equation (img 2) u [y, y], u [z, z] or simply y ^ 2, z ^ 2, Maple shows an empty string in Infinitesimal.

Thank you in advance.