code: function assertOffset(offset, min, cap, size) {
      if (typeof offset !== 'number' || offset % 1 !== 0)
          throw TypeError("Illegal offset: "+offset+" (not an integer)");
      offset = offset | 0;
      if (offset < min || offset > cap-size)
          throw RangeError("Illegal offset: "+min+" <= "+value+" <= "+cap+"-"+size);
      return offset;
  }
docs: Asserts that `min <= offset <= cap-size` and returns the type-safe offset.
  @param {number} offset Offset to assert
  @param {number} min Minimum offset
  @param {number} cap Cap offset
  @param {number} size Required size in bytes
  @returns {number} Type-safe offset
  @throws {TypeError} If `offset` is not an integer
  @throws {RangeError} If `offset < min || offset > cap-size`
  @inner