See
Kovács, L. G., Free Groupsgroups in a Dihedral Variety Ldihedral variety, Proc. GR. Kovács  Proceedings of the Royal Irish AcademyIr. Section A: Mathematical and Physical Sciences VolAcad. 89A, Sect. A 89, No. 1 (May, 1989), pp. 115-117 (1989). ZBL0697.20012.