function [y time]=umpasso(f,y0,t0,dt,nt)
time(1)=t0;
y(:,1)=y0;
for n=1:nt
 fn(:,1)=feval(f,y(:,n),time(n));
 y(:,n+1)=y(:,n)+dt*fn;
 time(n+1)=time(n)+dt;
endfor