for expressions comprising of a just monomial t we are always checking if a constant is a log derivative or not isn't it? 
in recognize_log_derivative [risch.py] we turn up with expression which are constant ( for such cases as q = a - zDd)  which needs a generator
but when i add a generator in reco_log_derivative i encounter unification errors for general cases that is the cases i have tested in test_risch.py