- fixed typos

- fixed typos
- degree(s), ldegree(s), and coeff(s) work if s is an ncmul and there are no
  other noncommutative factors in the target expression.
  Example: (42*T.a*T.b).coeff(T.a*T.b) returns 42, but (42*T.a*T.b).coeff(T.b)
  doesn't yield 42*T.a (coeff() is problematic with ncmul, anyway...)