Matrix trace

Solution P4.3.1

With a list comprehension, the matrix trace can be calculated with a single statement.

>>> M = [[1,2,3],
...      [4,5,6],
...      [7,8,9]]
...
>>> trace = sum(M[i][i] for i in range(3))
>>> print('The trace of M is', trace)
The trace of M is 15