I have the following result of expand grid:

> d <- expand.grid(c("x","y","z"),c("x","y","z"))

What I want is to create a combination of strings
but only the half of the all combinations:

  Var1 Var2
1    x    x
2    y    x
3   y    y
4   z    y
5   x    z
6    z    z


What's the way to do it?

G.V.

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