La traza de una matriz cuadrada es la suma de los valores de
la diagonal principal
Diseñad una función traza(mat) que, dada una matriz cuadrada mat representada
como una lista de listas, devuelva el valor de su traza.
About statements
The official statement of a problem is always the one
in the PDF document. The HTML version of the statement
is also given to help you, but may contain some content
that is not well displayed. In case of doubt, always use the PDF.