Fix for SF bug [ #471928 ] global made w/nested list comprehensions The symbol table pass didn't have an explicit case for the list_iter node which is used only for a nested list comprehension. As a result, the target of the list comprehension was treated as a use instead of an assignment. Fix is to add a case to symtable_node() to handle list_iter. Also, rework and document a couple of the subtler implementation issues in the symbol table pass. The symtable_node() switch statement depends on falling through the last several cases, in order to handle some of the more complicated nodes like atom. Add a comment explaining the behavior before the first fall through case. Add a comment /* fall through */ at the end of case so that it is explicitly marked as such. Move the for_stmt case out of the fall through logic, which simplifies both for_stmt and default. (The default used the local variable start to skip the first three nodes of a for_stmt when it fell through.) Rename the flag argument to symtable_assign() to def_flag and add a comment explaining its use: The third argument to symatble_assign() is a flag to be passed to symtable_add_def() if it is eventually called. The flag is useful to specify the particular type of assignment that should be recorded, e.g. an assignment caused by import.