Numbers in each arrow must be strictly increasing in the direction of the arrow.

In other words, if number "X" appears in an arrow, the numbers appearing after "X", must be greater than "X"; similarly

the numbers appearing before "X", must be smaller than "X".

You can construct a lot of rules, for examples number "9" cannot appear in the middle or beginning of

an arrow and number "1" cannot appear in the middle or the end of an arrow.

Here some youtube videos explaining it:

- https://youtu.be/mUDVgSm1HjQ

https://youtu.be/oJ9359S9tYY

https://youtu.be/2i8HmMLHS3o

https://youtu.be/yLEjzuOH40c