>>> from punt2D import Punt2D
>>> from poligon import Poligon
>>> from area import area
>>> pol = Poligon()
>>> area(pol)
0
>>> p = Punt2D(0, 0)
>>> pol.afegir_vertex(p)
>>> p = Punt2D(1, 0)
>>> pol.afegir_vertex(p)
>>> p = Punt2D(1, 1)
>>> pol.afegir_vertex(p)
>>> p = Punt2D(0, 1)
>>> pol.afegir_vertex(p)
>>> print('{:.2f}'.format(area(pol)))
1.00
>>> pol[2].y = 2
>>> print('{:.2f}'.format(area(pol)))
1.50
>>> pol[1].y = -2
>>> print('{:.2f}'.format(area(pol)))
2.50
>>> pol.afegir_vertex(Punt2D(-2, 0))
>>> print('{:.2f}'.format(area(pol)))
3.50