gensym pragma with names of entity passed as template parameter not working as expected

[cjxgm]

    template foo(txn, body: untyped): untyped =
        let txn {.gensym.} = 1
        body
    
    foo hello:
        echo hello
    
    foo hello:          # Error: redefinition of hello
        echo hello
    

How do I make the name of entity passed as a template parameter a `gensym` 
instead of `inject`? The `gensym` pragma seems not working.

I want to mimic the scoping behavior of a `for` statement. That is, with `for 
hello in ...: body`, the variable `hello` is only accessible in the `body`.