replacing the variables

["Min, Dughong" <min@xxxxxxxxxxxxxx>]

Hello, Ferret Users;
It's simple question. I am trying to replace a defined variable into
another expression like:

let mixed_den = density[z=0] - 0.15
let mixed_dep = density[z=@loc:mixed_den] 

The question is that i don't know how to replace the variables, mixed_den
with the [z=@loc:?] in the second line.  I tried it in several ways like
[z=@loc:mixed_den], z=@loc:`mixed_depth`],  or [z=@loc:"mixed_depth"].
 All thing that i had tried showed the error messages up. Please help me !

Thans for your help in advance.

Duke Min