anchor.diff:    /// `127`.
     ///
     /// If the input value fits in [width] bits without truncation, the result is
  -  /// the same as the input.  The minimum width needed to avoid truncation of `x`
  +  /// the same as the input. The minimum width needed to avoid truncation of `x`
     /// is `x.bitLength + 1`, i.e.
     ///
     /// ```dart
  
positive.code: ///
  /// Big integers are signed and can have an arbitrary number of
  /// significant digits, only limited by memory.
  abstract class BigInt implements Comparable<BigInt> {
    /// A big integer with the numerical value 0.
    external static BigInt get zero;
  
negative.code:   ///
    /// Throws a [FormatException] if the [source] is not a valid integer literal,
    /// optionally prefixed by a sign.
    /// Examples:
    /// ```dart
    /// print(BigInt.parse('-12345678901234567890')); // -12345678901234567890
    /// print(BigInt.parse('0xFF')); // 255
    /// print(BigInt.parse('0xffffffffffffffff')); // 18446744073709551615
    ///
    /// // From binary (base 2) value.
    /// print(BigInt.parse('1100', radix: 2)); // 12
    /// print(BigInt.parse('00011111', radix: 2)); // 31
    /// print(BigInt.parse('011111100101', radix: 2)); // 2021
    /// // From octal (base 8) value.
    /// print(BigInt.parse('14', radix: 8)); // 12
    /// print(BigInt.parse('37', radix: 8)); // 31
    /// print(BigInt.parse('3745', radix: 8)); // 2021
    /// // From hexadecimal (base 16) value.
    /// print(BigInt.parse('c', radix: 16)); // 12
    /// print(BigInt.parse('1f', radix: 16)); // 31
    /// print(BigInt.parse('7e5', radix: 16)); // 2021
    /// // From base 35 value.
    /// print(BigInt.parse('y1', radix: 35)); // 1191 == 34 * 35 + 1
    /// print(BigInt.parse('z1', radix: 35)); // Throws.
    /// // From base 36 value.
    /// print(BigInt.parse('y1', radix: 36)); // 1225 == 34 * 36 + 1
    /// print(BigInt.parse('z1', radix: 36)); // 1261 == 35 * 36 + 1
    /// ```
    external static BigInt parse(String source, {int? radix});
  
    /// Parses [source] as a, possibly signed, integer literal and returns its
  