/* presentation on standard generators for J1 */

PresentationJ1 := function (G : UserGenerators := [], Projective := false)
   Fct := Projective select ProjectiveOrder else Order;
   if #UserGenerators eq 0 then 
      UserGenerators := [G.i : i in [1..Ngens (G)]];
   end if;
   if #UserGenerators ne 2 then "Require 2 user generators"; 
                           return false; end if;
   a := UserGenerators[1]; b := UserGenerators[2];
   return Fct (a) eq 2 and Fct (b) eq 3 and Fct (a * b) eq 7
   and Fct (a*b*(a*b*a*b^-1)^3) eq 5 and Fct (a*b*(a*b*a*b^-1)^6*a*b*a*b*(a*b^-1)^2) eq 2;
end function;