1 | initial version |

It is, i think, the following function `f`

...

```
def g(x): return abs( 2*RR( abs(x)/2 ).frac() - 1 )
def f(x): return pi * g(x/pi)
```

In a picture...

```
plot( g, -3, 3, aspect_ratio=1 )
plot( f, -3*pi, 3*pi, aspect_ratio=1 )
```

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