Issue #974: Add an assertion to Attr.visitLetExpr() so we know if we try to enter variables into a DelegatedScope (a DelegatedScope is 'An empty scope, into which you can't place anything')
Issue #974: Stop generating local anonymous subclasses of AbstractIterator and use named local subclass, so we can use a constructor to initialise the iter$0 field, so that if the iterator transforms to a Let the Let has a scope to operate on.
Issue 974: Tidy up test, and remove _fail
Issue #974: Fix the .src for the modified Comprehension transformation
Issue #985: Add a test