## Takes in the previous and the present profiles and iterates to find
## the profile in the next time step.

function ynext=propagate(ynow,yprev,r)

  ## Quick and dirty way to fix boundary conditions -- for each step
  ## they are the same as the previous step.

  ynext=ynow;
  for i=2:length(ynow)-1
    ynext(i) = 2*(1-r^2)*ynow(i)-yprev(i)+r^2*(ynow(i+1)+ynow(i-1));
  endfor
  
endfunction