+ # The element of each type
+ var element_mtype: nullable MType
+
+ # Set that `self` is a part of comprehension array `na`
+ # If `self` is a `for`, or a `if`, then `set_comprehension` is recursively applied.
+ private fun set_comprehension(n: nullable AExpr)
+ do
+ if n == null then
+ return
+ else if n isa AForExpr then
+ set_comprehension(n.n_block)
+ else if n isa AIfExpr then
+ set_comprehension(n.n_then)
+ set_comprehension(n.n_else)
+ else
+ # is a leave
+ n.comprehension = self
+ end
+ end