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: selection_set.rs
    path of file: ./repos/grafbase/engine/crates/engine-v2/src/request/walkers
    
    the code of the file until where you have to start completion: use schema::{Definition, DefinitionWalker};

use super::{BoundFieldWalker, BoundFragmentSpreadWalker, BoundInlineFragmentWalker, OperationWalker};
use crate::request::{BoundSelection, BoundSelectionSetId, SelectionSetType};

pub type BoundSelectionSetWalker<'a> = OperationWalker<'a, BoundSelectionSetId>;
pub type SelectionSetTypeWalker<'a> = OperationWalker<'a, SelectionSetType, Definition>;

impl<'a> BoundSelectionSetWalker<'a> {
    pub fn ty(&self) -> SelectionSetTypeWalker<'a> {
        let ty = self.as_ref().ty;
        self.walk_with(ty, Definition::from(ty))
    }

    pub fn fields(self) -> Selection
    
    fn next(&mut self) -> Option<Self::Item> {
        loop {
            let selections = self.selections.last_mut()?;
            if let Some(selection) = selections.next() {
                match selection {
                    BoundSelectionWalker::Field(field) => return Some(field),
                    BoundSelectionWalker::InlineFragment(inline) => {
                        self.selections.push(inline.selection_set().into_iter());
                    }
                    BoundSelectionWalker::FragmentSpread(spread) => {
                        self.selections.push(spread.selection_set().into_iter());
                    }
                };
            } else {
                self.selections.pop();
            }
        }
    }