Correction of DE in test_rde.py
DE correction in test_risch.py
Chanes in cds_cancel_exp
Works for the following cases:

pa: Poly(0, t2, domain='ZZ(x)')
pd: Poly(x**3*t2**2, t2, domain='ZZ(x)')

pa: Poly(t2**2 + 1, t2, domain='ZZ(x)')
pd: Poly(t2*x, t2, domain='ZZ(x)')