function foo(itemstack, placer, pointer)
    local vm = minetest.get_voxel_manip()

    local rp1 = {
        x = pointer.above.x - 10,
        y = pointer.above.y - 10,
        z = pointer.above.z - 10
    }

    local rp2 = {
        x = pointer.above.x + 10,
        y = pointer.above.y + 10,
        z = pointer.above.z + 10
    }

    local c_cobble = minetest.get_content_id("default:mossycobble")

    local ep1, ep2 = vm:read_from_map(rp1, rp2)
    local data = vm:get_data()

    local a = VoxelArea:new{
        MinEdge={x=ep1.x, y=ep1.y, z=ep1.z},
        MaxEdge={x=ep2.x, y=ep2.y, z=ep2.z},
    }

    for z = rp1.z, rp2.z do
    for y = rp1.y, rp2.y do
    for x = rp1.x, rp2.x do
        local x0 = x - (rp1.x + rp2.x) / 2
        local y0 = y - (rp1.y + rp2.y) / 2
        local z0 = z - (rp1.z + rp2.z) / 2
        if x0*x0 + y0*y0 + z0*z0 <= 25 then
            local i = a:index(x, y, z)
            data[i] = c_cobble
        end
    end
    end
    end

    vm:set_data(data)
    vm:write_to_map()
    vm:update_map()

    return itemstack
end

minetest.register_craftitem("default:test", {
    inventory_image = "default_tool_steelaxe.png",
    on_place = foo
})