Defining an implicit function

I'm not sure if this is possible, but it is certainly in the spirit of ImplicitCAD, so I would like to know if anyone knows how to do it. A Haskell answer is just as good as an extopenscad answer.

Suppose I want to define a new implicit function, in this case, the astroid. It is similar to the circle, but has a different L-norm.

```
astroid(x,y) = (x**(2/3)+y**(2/3))**(3/2) -1
```

This function is negative for (x,y) in the astroid region, zero on the perimeter and positive outside. I would now like to use this function in ImplicitCAD in this manner (replacing circle(1) with the new astroid shape):

This works:
```
linear_extrude(height(x,y) = 1 + cos(x)/2 + cos(y)/2 )
circle(1);
```

I would like to make this:
```
linear_extrude(height(x,y) = 1 + cos(x)/2 + cos(y)/2 )
astroid;
```