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: formula.rs
    path of file: ./repos/CreuSAT/CreuSAT/src
    
    the code of the file until where you have to start completion: // Formula is Mac OK with an inline_full + split on VC #12 for add_clause 11.04 22.18
extern crate creusot_contracts;

use creusot_contracts::{std::*, Ghost, *};

use crate::{assignments::*, clause::*, 
    
    fn delete_clause(&mut self, cref: usize, watches: &mut Watches, t: &Trail) {
        let old_f: Ghost<&mut Formula> = ghost! { self };
        watches.unwatch(self, t, cref, self.clauses[cref][0]);
        watches.unwatch(self, t, cref, self.clauses[cref][1]);
        self.clauses[cref].deleted = true;
        proof_assert!(forall<i: Int> 0 <= i && i < self.clauses@[cref@]@.len() ==>
            self.clauses@[cref@]@[i] == old_f.clauses@[cref@]@[i]);
        proof_assert!(old_f.equisat(*self)); // This assertion helps with the invariant, which otherwise takes a long time.
        proof_assert!(^self == ^old_f.inner());
    }