```
f <- function(x, y) x^2 - y^2
xx <- c(-1, 1); yy <- c(-1, 1)
## Not run:
# vectorfield(f, xx, yy, scale = 0.1)
# for (xs in seq(-1, 1, by = 0.25)) {
# sol <- rk4(f, -1, 1, xs, 100)
# lines(sol$x, sol$y, col="darkgreen")
# }
# grid()## End(Not run)
```

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