Gmpl How can I do this?

Hello

I am trying to do some simple gmpl model but as a complete beginner I noticed I cannot do it.

    col1 col2 col3
row1  x
row2  x    x
row3  x    x    x

Assign numbers 1..6 to the x cells so that every number has different row and col than the previous number.

I tried to look sudoku.mod and some other examples but those were too difficult to understand for now.

Br, Mikko