All Fib elements except the root-element will be introduced by a (at least four bit) bit field. Vectors are directly included in the Fib elements, without ther own introduction.
The introduction is reserved, so that with 0 padded areas can not be interpreted as a Fib element. The introduction announces that the introduction is at least 16 bits long and not 4 bits.
All Fib elements and vector elements follow directly behind each other. If positions are padded with the 0 bit, this is explicitly stated hereafter.