// Phase 200-B: Minimal parse_number with digits capture
static box Main {
    main() {
        local s = "42abc"
        local digits = "0123456789"  // ← Captured var

        local p = 0
        local num_str = ""
        local n = s.length()

        loop(p < n) {
            local ch = s.substring(p, p+1)
            local digit_pos = digits.indexOf(ch)  // ← Uses captured digits

            if digit_pos < 0 {
                break
            }

            num_str = num_str + ch
            p = p + 1
        }

        print(num_str)  // Expected: "42"
    }
}