# Simplifying a proof in our book

In [0], pp.66-69, we show how the conditional distribution of  ∧  over  ∀  can be derived from the one-point rule and the other axioms. I dragged sets into the picture, for which misbehaviour I apologize; here is a simpler argument. Representing “the non-empty range” for the dummy   by r.x  ∨  [x=y] we show

[〈∀x : r.x ∨ [x=y]: t.xQ〉 ≡ 〈∀x : r.x ∨ [x=y]: t.x〉∧ Q ]

To this end we observe

〈∀x: r.x ∨ [x=y]: t.xQ

=         {splitting the term}

〈∀x: r.x ∨ [x=y]: t.x〉 ∧ 〈∀x: r.x ∨ [x=y]: Q

=         {see (*) below}

〈∀x: r.x ∨ [x=y]: t.x〉 ∧ Q

(*) We observe

〈∀x: r.x ∨ [x=y]: Q

=         {splitting the range}

〈∀x: r.x: Q〉∧〈∀x: [x=y]: Q

=         {one-point rule}

〈∀x: r.x: Q〉∧ Q

=         {see (**) below)

Q

(**) We observe

[Q ⇒〈∀x:r.x: Q〉]

=         { ⇒ distributes —like ∨— over ∀ in consequent}

[〈∀x: r.x: QQ〉]

=         {pred. calc}

[〈∀x: r.x: true〉]

=         {pred. calc., e.g. [0], p.66, (90)]

[true]

=         {pred. calc.}

true

[0] Edsger W. Dijkstra & Carel S. Scholten Predicate Calculus and Program Semantics, Springer-Verlag New York Inc., 1990

Austin, 25 September 1991

prof.dr. Edsger W.Dijkstra
Department of Computer Sciences
The University of Texas at Austin
Austin, TX 78712–1188
USA