Defining nested dynamic variables under :dynamic_variables header in DSL

[neonWhiteout]

@stock_and_flow begin
:stocks
A
B

   :dynamic_variables
   v = (A + A) / B
   end

fails, when we'd expect it to create an implicit v0 = A + A and then define v = v0 / B

[neonWhiteout]

@Saityi