References
  
  [CLLT93]  Chan,  S.-P.,  Lang,  M.-L.,  Lim,  C.-H.  and Tan, S. P., Special
  polygons  for  subgroups of the modular group and applications, Internat. J.
  Math., 4, 1 (1993), 11--34.
  
  [Kul91]  Kulkarni, R. S., An arithmetic-geometric method in the study of the
  subgroups of the modular group, Amer. J. Math., 113, 6 (1991), 1053--1133.
  
  [LLT95a]  Lang,  M.-L.,  Lim,  C.-H.  and  Tan,  S.  P.,  An  algorithm  for
  determining  if  a  subgroup  of  the modular group is congruence, J. London
  Math. Soc. (2), 51, 3 (1995), 491--502.
  
  [LLT95b]  Lang, M.-L., Lim, C.-H. and Tan, S. P., Independent generators for
  congruence subgroups of Hecke groups, Math. Z., 220, 4 (1995), 569--594.