def index(fks, id):
    """Looks for the id on fks, fks is an array of arrays, each array has on [1]
    the id of the class in a dia diagram.  When not present returns None, else
    it returns the position of the class with id on fks"""
    for i, j in fks.items():
        if fks[i][1] == id:
            return i
    return None