Makkai workship Birthday offering
  Billed to the organisers as “a short talk ... offering a peculiar topological space that Peter Johnstone and I used some 30 years ago to solve a topoidal problem.”

The space? — Heath’s V-space, a re-topologised real upper half-plane. (Actually: the quotient space of R_discrete × R_metric under the equivalence relation (x-y, y) ~ (x+y, -y) , the metric on the second factor being given by: d(x, y) = (1-δ_x,y)·max{|x|, |y|} .) [>>]