The routine runge is a simple Runge-Kutta stepper.
It does not incorporate accuracy estimates or step size adjustment.
The call to runge is
runge (n, h, t, y, der)
n (int) is the number of ODE's
h (float) is the time step
t (float) is time at the start of the step
y ( float array) is the array of dependent variables at time t
and is modified to be the array of dependent variables at time t+h.
der is the name of a subroutine that will calculate the derivatives
of y (the 
above). der will be called by runge. You must write
this subroutine.
It is in this subroutine that the form of ODE's is specified.
Each call to runge advances the solutions y by a step h in
time, provided your der correctly specifies the ODE's.
The call to der must be as follows:
der (t, y, dydt)
t (float) is current time
y (float array) is the solution as above
dydt (float array of derivatives of y at time t (this is how the
ODE's are specified)).