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    use std::ops::{AddAssign, MulAssign};

    // For implementing base10_digits() accessor on LitInt.
    pub struct BigInt {
        digits: Vec<u8>,
    }

    impl BigInt {
        pub fn new() -> Self {
            BigInt { digits: Vec::new() }
        }

        pub fn to_string(&self) -> String {
            let mut repr = String::with_capacity(self.digits.len());

            let mut has_nonzero = false;
            for digit in self.digits.iter().rev() {
                has_nonzero |= *digit != 0;
                if has_nonzero {
                    repr.push((*digit + b'0') as char);
                }
            }

            if repr.is_empty() {
                repr.push('0');
            }

            repr
        }

        fn reserve_two_digits(&mut self) {
            let len = self.digits.len();
            let desired =
                len + !self.digits.ends_with(&[0, 0]) as usize + !self.digits.ends_with(&[0]) as usize;
            self.digits.resize(desired, 0);
        }
    }

    impl AddAssign<u8> for BigInt {
        // Assumes increment <16.
        fn add_assign(&mut self, mut increment: u8) {
            self.reserve_two_digits();

            let mut i = 0;
            while increment > 0 {
                let sum = self.digits[i] + increment;
                self.digits[i] = sum % 10;
                increment = sum / 10;
                i += 1;
            }
        }
    }

    impl MulAssign<u8> for BigInt {
        // Assumes base <=16.
        fn mul_assign(&mut self, base: u8) {
            self.reserve_two_digits();

            let mut carry = 0;
            for digit in &mut self.digits {
                let prod = *digit * base + carry;
                *digit = prod % 10;
                carry = prod / 10;
            }
        }
    }