degree(), ldegree(), coeff(), lcoeff(), tcoeff() and collect() can now
be used with constants and indexed expressions as well, so you can use
it to collect by powers of Pi or find the coefficient of gamma~0.
Limitations:
- it only works with symbols, constants and indexed expressions;
trying to find the coefficient of, e.g., "x^2" or "x+y" won't work
- it does not know about dummy index summations; the coefficient of
gamma~0 in p.mu*gamma~mu should be p.0 but is returned as 0
- using the functions on elements of noncommutative products might
return wrong or surprising results