How can we solve two 1st- order ODE equations simultaneously

Given two 1st order ODE equations

dy/dt = z

dz/dt = u(1-y^2)z - y

Two 1st order ODE above are coupled and must be solved simultaneously. (First iteration will solve for ý and z1 using inital conditions y0 and z0, and second inter solve y2 z2 using y1 z1)

How can we write function fiel to perform Euler's method to solve ?