What's "P and L"?

For the Lorenz curve of income distribution, gini index derives from P and L where:
    P: cumulated % of individuals
    L: cumulated % of income

and the step-by-step calculation I was talking about looks like:

scalar area = 0
scalar gini = 0
loop i = 2..$nobs --quiet
    area = (L[i] + L[i-1])*(P[i] - P[i-1]) + area     # the area under the Lorenz curve
endloop
area = (area + P[1]*L[1])/2
gini = (1 - 2 * area)

The very small difference came from the additional last (and pretty small) term I missed : P[1]*L[1]
best,
artur