function test_rider_leads_horses()
    local r0 = region.create(0, 0, "plain")
    local r1 = region.create(1, 0, "plain")
    local r2 = region.create(2, 0, "plain")
    local r3 = region.create(3, 0, "plain")
    local f = faction.create("human")
    -- lead 1 extra horse per level while riding:
    local u1 = unit.create(f, r0, 1)
    u1:set_skill("riding", 1)
    u1:add_item("horse", 2)
    u1:add_order("NACH O O O")
    -- too heavy to ride, walk the horses:
    local u2 = unit.create(f, r0, 1)
    u2:set_skill("riding", 1)
    u1:add_item("horse", 2)
    u2:add_item("money", 2000 * 2)
    u2:add_order("NACH O O")

    process_orders()
    print(u1.region.x, u2.region.x)
    assert_equal(r1, u2.region)
    assert_equal(r2, u1.region)
end