Helmholtz equation syntax question (Does "A_t" represent time derivative?)

In one of your examples, under "Classic PDE" > "Interference and Diffraction",  you say you're using the Helmholtz equation in the documentation, but then you define:
    

    -(Ax_x + Ay_y) - k^2*A_t = 0

I think "A_t" is the time derivative, so why is that in there? Shouldn't it just be "A"?

No, "A_t" designates "A*test_function_for_A" while "A'" (A + single quote) is the time-derivative. Furthermore "A_t" is here used to assemble the term implicitly (to the system matrix), instead of explicitly (to the RHS).

See the table listing for the custom physics mode for more information on the equation syntax of FEATool: https://featool.com/doc/physics#phys_ce