35 ;;Quote: type system for reference immutability; i.e., reachable state not modified through the reference; dynamic checking for downcasts; parameterization

36 ;;Quote: if a return result is readonly, can safely return the representation type; e.g., the array for an IntSet

36 ;;Quote: use 'readonly' for immutabile constraints and 'mutable' for supplementary fields and explicit downcasts; a mutable field is not part of the abstract state

37 ;;Quote: use 'mutable' to downcast a read-only reference to a non-read-only reference; inserts run-time checks to enforce immutability

37 ;;Quote: distinguish an array of read-only references from a read-only array

38 ;;Quote: a mutable, readonly field may be assigned to a readonly object; a mutable, final field may be modified but not assigned; a read-only, final field may not be changed

43 ;;Quote: writing new code is natural with reference immutability; helped define interfaces; easier than annotating existing code

43 ;;Quote: reference immutability is more effective than object immutability; avoids performance cost of immutable objects

43 ;;Quote: with parent and child links, transitive immutability may be too strong; one reference can make the entire container immutable; e.g., a UI component

44 ;;Quote: added readonly annotations at 1500 lines/hour; found 19 errors, e.g., sorting the input array