Since he has just passed away, I was trying to find a free PDF link of my favorite Hoare paper, Proof of Correctness of Data Representations, but unfortunately it is under the Springer iron dome. You can enter the DOI 10.1007/3-540-07994-7_54 into your preferred website for liberating scientific information.
@sree I already checked that before posting; it's not under open access.
@pervognsen Gah! I saw free access, and I thought ACM had released all older papers open access. Apparently not, sorry!
@sree @pervognsen they only released older research papers, which they own copyright to. This is from a book they likely do not own the copyright to.