[ferret_users] Define new variable!

["Yu, Hao-Cheng" <danish.url@xxxxxxxxx>]

Dear ferret users,

I've been trying to define a new variable and use this variable as index of i-axis, like

--------------------------
let e1=nv[l=1]
let elex=lon[i=e1]
--------------------------

"nv" is integer, define i-index,
"lon" is longitude (also real)
I try to get "lon" variable at specific i-index.
The first line, e1 is OK.
But the second not work, ferret tells me "**ERROR: command syntax:"

ferret seems not support this kind of using!
Any suggestions?