我要用两个循环，需要1-1对应。
local fish1 carp bass trout
local fish2 catfish sword shark
foreach i of local fish1 {
foreach j of local fish2 {
gen diff_`i'_`j'=y`i'-y`j'
}
}
可是这样我就有9个diff. 我要3个diff，carp-catfish, bass-sword, trout-shark. 请帮忙改一下，多谢。