input_case nil$

% Matrix element of chi -> gg
% ---------------------------
% quark mass = 1; n - unit vector along the decay axis
% k1, k2, e1, e2 - gluon momenta and polarizations
mass p=1,n=i; mshell p,n; vector k,k1,k2,ee,e1,e2,e0;
let p.n=0,p.k=0,p.ee=0,p.e1=0,p.e2=0,n.e1=0,n.e2=0;
k1:=p+n$ k2:=p-n$
operator r,S; for all p let r(p)=g(f,p)+1,S(p)=(g(f,p)+1)/(p.p-1);
M:=g(f,e2)*S(p+k-k1)*g(f,e1)*r(p+k)*g(f,ee)*r(-p+k)$
for all p clear r(p),S(p); clear r,S;
M:=M+sub(e0=e2,sub(e2=e1,sub(n=-n,e1=e0,M)))$
M:=sub(x=0,df(sub(k=x*k,M),x)); % linear approximation

end;