Block Number declaration in Scilla

gistfile1.txt

contract Block()
 
field a: Option BNum = None {BNum} (*declaring variable a with no value*)
field b : BNum = BNum 5            (*declaring variable b with some fixed value*)

transition update_value()

   (*updating variable a*)
   c = BNum 6; (*declaring variable c with fixed value in the transition*)
   tmp = Some {BNum} c; 
   a:= tmp;
    
   (*updating variable b*)
   d= BNum 7; (*declaring variable d in the transition*)
   b:= d
    
end