st: looping twice over same variables without repeating results

[Beatriz Rodriguez Prado <bprado@eco.uva.es>]

Hello,
I need some help with this situation.
I have 3 variables named pmm_and pmm_ext pmm_es and I want to make 
simple regressions of each pair of variables.
I have writen this code

foreach x in and ext es {
    foreach y in and ext es{
        if pmm_`x'!=pmm_`y' {
        regres pmm_`x' pmm_`y'
        }
    }
}

How could avoid to run repeated regressions?
In fact I have 50 variables and have to run regressions, test some 
hyphotesis and make graphs and the output of the loop is unnecesary big.
Thank you very much
Beatriz Rodriguez
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