[R] Defining a Design Matrix for Correlation

[L.E.Gross]

Dear list

I am using the 'geese' command in the geepack Package add-on to R.

I am wanting to define my own correlation matrix for 4 time occasions 
such that there is correlation between times 1&2, 1&3, 2&4 and 3&4 but 
not 1&4 and 2&3.

There is an option to define a design matrix called zcorr. Could anyone
tell me how I can create this design matrix such that it will show the 
correlations that I need to include?

Many thanks
Laura
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