function phaseportpar(eqns, timespan, plotrange, solver, par)

clf;
axis(plotrange);
hold on;
button = 1;
while button==1,
[xinit(1), xinit(2), button] = ginput(1);
if button ~= 1 break; end;
[T,Y] = feval(solver, eqns, timespan, xinit, [], par);
plot(Y(:,1), Y(:,2));
[T,Y] = feval(solver, eqns, -timespan, xinit, [], par);
plot(Y(:,1), Y(:,2));
end;
