Yes tuples are allowed and there is no need for double `(())` as it's ugly.
How about this syntax:
case mytree
of StrValue as s: # if s is of type string
of Minus as m: # then m is of type UnaryOp
# or
of Minus as (m): # unpacking, m is of type Tree
of IfStmt as (cond, le, ri): # unpacking
Run
