Internal gears

[Equidamoid]

Is your feature request related to a problem? Please describe.
Let's say I want to make a planetary gearbox. How to generate the outer ring (with the teeth pointing inside)?

Describe the solution you'd like
Flip the teeth automatically when hole_diameter is greater than pitch_radius() for this gear.

Describe alternatives you've considered
Add inner=false parameter to gear(). May be easy to confuse with interior=.

[revarbat]

The interior=true argument to gear() is key here:

difference() {
    cylinder(h=10,d=120,center=true);
    gear(mm_per_tooth=10, number_of_teeth=32, thickness=11, interior=true);
}