XOR (r as boolean) standard constraint

def xors_constraint(v=1):
    """ XOR (r as boolean) standard constraint"""
    assert v in [0,1], "v must be 0 or 1 instead of %s" % v.__repr__()
    model, x, y, z = _init()
    r = True
    model.addConsXor([x,y,z], r)
    model.addCons(x==v)
    _optimize("Standard XOR (as boolean)", model)