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