ACM Sigplan Notices 27, 8 (Aug. 1992), 89-98.Introduction Neither a borrower nor a lender be; For loan oft loses both itself and friend ... [Shakespeare, Hamlet, I. iii 75] The sharing of data structures can be efficient, because sharing can substitute for copying, but it creates ambiguity as to who is responsible for their management. This ambiguity is difficult to statically remove [Barth77] [Bloss89] [Chase87] [Hederman88] [Inoue88] [Jone
