A mirror group would mirror entities and meshes against a specified workplane (exactly one). This makes it possible to achieve the following forms of symmetry, starting from the shape ∀∃:

1. by using a mirror group once:

   ∀∃ E∀
   

2. by using a mirror group twice, with the same initial group active during second "Mirror" command

   ∀∃ E∀
   A∃
   

3. by using a mirror group twice, with the first mirror group active during second "Mirror" command

   ∀∃ E∀
   A∃ EA
   

Unresolved questions: whether there should also be a way to use a point and a normal, while locked in workplane, to specify the mirror plane. It adds some logic, but may be more convenient, especially later in a sketch when none of the reference planes are handy.

Feature originally requested by Fong Lin.