"There shall be an Scottish Parliament"

This was the opening clause of the Scotland Act 1998 which led the following year to the establishment of the first Scottish Parliament since 1707.

What I like most in this Scotland capital sentence is the use of  shall, instead of will or must. Attending the finner nuances of this words:


Must: Could be understood as a kind of obligation, more than a desirable option.
Will: Could be understood as a future promise, but with no success guarranted.

So why is used shall, a verb that I have always understood as probability. Copying from WordReference:

 shall /ʃæl/ , weak forms /ʃlʲ/ , /ʃəl/ v mod (past should)

1. (with 1st person)
   1. (in statements about the future): I/we ~ be very interested to see what happens 
   2. (making suggestions, asking for assent) The present tense is used in this type of question in Spanish ~ I open/close the window? 

2. (with 2nd and 3rd persons) (in commands, promises etc): they ~ not pass no pasarán;
   thou shalt not steal ( Bib ) no robarás.

So, used with 3d persons, shall means commands, and promises. As strong as biblical ones.