You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: nnf.rs
    path of file: ./repos/rusty-razor/razor-fol/src/transform
    
    the code of the file until where you have to start completion: /*! Defines formulae in Negation Normal Form (NNF) and implements an algorithm for
transforming an [`FOF`] to an [`NNF`].

[`FOF`]: crate::syntax::FOF
*/
use crate::syntax::{
    formula::{clause::Literal, *},
    term::Complex,
    Error, Sig, Var, FOF,
};

/// Represents a formula in Negation Normal Form (NNF).
///
/// **Hint**: An NNF is a formula where
    
    fn nnf_transform() {
        let nnf = fof!({!x. {? y. {[P(f(x), y)] | [~(Q(z))]}}} & {[~(R(f(x), z))] | [R(@c)]}).nnf();
        let f = |t: &Complex| {
            {
                if t == &term!(f(x)) {
                    term!(z)
                } else {
                    t.clone()
                }
            }
            .into()
        };
        assert_eq!(
            fof!({!x. {? y. {[P(z, y)] | [~(Q(z))]}}} & {[~(R(z, z))] | [R(@c)]}),
            FOF::from(nnf.transform_term(&f))
        );
    }