code: ByteVector put11(final int b1, final int b2) {
          int length = this.length;
          if (length + 2 > data.length) {
              enlarge(2);
          }
          byte[] data = this.data;
          data[length++] = (byte) b1;
          data[length++] = (byte) b2;
          this.length = length;
          return this;
      }
docs: Puts two bytes into this byte vector. The byte vector is automatically
  enlarged if necessary.
  
  @param b1
  a byte.
  @param b2
  another byte.
  @return this byte vector.