// CM exclusions. 
   j := jInvariant(E);
   if j in [0, 2^4*3^3*5^3, -2^15*3*5^3] then
      B *:= 3;
   elif j in [2^6*3^3, 2^3*3^3*11^3] then
      B *:= 2;
   elif j in [-3^5*5^3, 3^3*5^3*17^3] then
      B *:= 7;
   elif j in [2^6*5^3] then
      B *:= 2;
   elif j in [-2^15] then
      B *:= 11;
   elif j in [-2^15*3^3] then
      B *:= 19;
   elif j in [-2^18*3^3*5^3] then
      B *:= 43;
   elif j in [-2^15*3^3*5^3*11^3] then
      B *:= 67;
   elif j in [-2^18*3^3*5^3*23^3*29^3] then
      B *:= 163;
   end if;