def diff_test(i):
 syms = i.free_symbols
 for s in syms:
  assert ((i.diff(s).doit() - i.doit().diff(s)).expand() == 0)
 return syms

def diff_test ( i ) : syms = i . free_symbols for s in syms : assert ( ( i . diff ( s ) . doit ( ) - i . doit ( ) . diff ( s ) ) . expand ( ) == 0 ) return syms
return the set of symbols .