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: symbol_visitor.rs
    path of file: ./repos/boa/examples/src/bin
    
    the code of the file until where you have to start completion: // This example demonstrates how to use a visitor to perform simple operations over the Javascript
// AST, namely: finding all the `Sym`s present in a script. See commuter_visitor.rs for an example
// which mutates the AST.

use boa_ast::visitor::Visitor;
use boa_engine::{Context, Source};
use boa_interner::Sym;
use boa_parser::Parser;
use core::ops::ControlFlow;
use std::{collections::HashSet, convert::Infallible, path::Path};

#[derive(Debug, Clone, Default)]
struct SymbolVi
    
fn main() {
    let mut parser = Parser::new(Source::from_filepath(Path::new("./scripts/calc.js")).unwrap());
    let mut ctx = Context::default();

    let script = parser.parse_script(ctx.interner_mut()).unwrap();

    let mut visitor = SymbolVisitor::default();

    assert!(matches!(
        visitor.visit_statement_list(script.statements()),
        ControlFlow::Continue(_)
    ));

    println!(
        "Observed {} unique strings/symbols:",
        visitor.observed.len()
    );
    for sym in visitor.observed {
        println!("  - {}", ctx.interner().resolve(sym).unwrap());
    }
}