2 2 50  Nc
  This  switch has two positions, "3" and "inf".
"3"  corresponds to normal SU(3) chromodynamic. 
If the switch is in the "inf" position then 
symbolic calculation occur for SU(Nc) color 
group and only the leading term of 1/Nc expansion
is saved.  Nc=3 is substituted into the  final 
answer.  This trick allows to remove a lot 
of interference diagram, which are zero in the
leading term of 1/Nc expansion.
