What theorem does this represent?

GeoGebra Commands & Parameters

1. Circle(A, B) → c
2. Tangent(C, c) → a, b
3. Distance(a, b) → d
4. Intersect(c, a, 1) → D
5. Segment(D, C) → e
6. Intersect(c, b, 1) → E
7. Segment(E, C) → f
8. Distance(E, f) → distanceEf