'For an associative word `self`, returns the number of letters in it.
Examples
>>> from sympy.combinatorics.free_groups import free_group
>>> f, a, b = free_group("a b")
>>> w = a**5*b*a**2*b**-4*a
>>> len(w)
13
>>> len(a**17)
17
>>> len(w**0)
0'

def __len__(self):

    return sum((abs(j) for (i, j) in self))
