get the 'up value of a variable across a lambda

[polifemo]

while
`(let @ 1 (let @ 10 (let @ 20 (up 2 @))))`
returns 1, as I expected, if I enclose the 'up expression inside a function
like `(de f (N) (up N @))` then no value of N will yield the value of @ in
an environment higher than the innermost let:
`
: (let @ 1 (let @ 10 (let @ 20 (f 3)))
-> 20
`
how can I access the (@ . 1) value in that higher environment from inside
the function?