System:
    iter0 = startScan(Result)
    while (Tup = nextTuple(iter0)) { display Tup }
    endScan(iter0)
Result:
    iter1 = startScan(Join)
    while (T = nextTuple(iter1))
        { T' = project(T); return T' }
    endScan(iter1)
Sel1:
    iter4 = startScan(Btree(Enrolment,'semester=05s2'))
    while (A = nextTuple(iter4)) { return A }
    endScan(iter4)
...