#### Reading material

Pages 115-119.
#### Additional material

A draft version of a note on Hoare's proof system:
PostScript and PDF.
This note will hopefully be expanded soon.
#### Question

Give a proof using Hoare's proof system of
{ y = oldY }
x = x + y;
y = y - x;
if (x + y > 0)
y = x + y;
else
y = -x - y;
{ x = oldX + oldY and ((oldY > 0 and y = oldY) or (oldY <= 0 and y = -oldY)) }