\starttext

\placeregister
  [index]
  [n=1]

\blank

One
    \startregister[index][mytilene]{Mytilene}
    \index{Athens}Athens
    \index{athenian}athenian
    \index{aardvark}aardvark
    \index{Azygous}Azygous
    \dorecurse{15}{\input tufte}
    \stopregister[index][mytilene]
Two

\stoptext
