  If there is one polygon inscribed into some smooth conic and circumscribed
about another one, then there are infinitely many such polygons. This is
Poncelet's theorem. The aim of this note is to collect some (mostly classical)
versions of this theorem, namely:
  - Weyr's Poncelet theorem in $P_3$ (1870),
  - Emch's theorem on circular series (1901),
  - Gerbaldi's formula for the number of Poncelet pairs (1919),
  - the Money-Coutts theorem on three circles (1971),
  - the zig-zag theorem (1974),
  - a (probably new) Poncelet theorem on three conics,
  - a Poncelet formula for quadrics of revolution.
