How to export multiple circles?

I am trying to export multiple circles with different radii to STEP format. When I import the STEP file into my CAD software I see only one circle (the last one created). Here is the minimal code to reproduce:

```
import cadquery as cq

wp = cq.Workplane("XY")
wp = wp.circle(1)
wp = wp.circle(2)
cq.exporters.export(wp, "circles.step")
```

Help would be appreciated!
