The filter function extra arguments are handled in a fexpr-like way (ancient Lisp terminology). They are treated as source code, and evaluated as many times as the filter is called. They should be evaluated once, at the time the filter expression is reduced to a filter, and only the resulting values captured.

This is not entirely trivial because the v_fun function itself is a special operator that expects unevaluated forms. Double evaluation would be even more wrong.

One hack is to create some gensyms and bind the evaluated
argument values to them in the binding environment, and pass the
gensyms instead of the original arguments.