composer index&{s:}
  @index: 0;
  [<match>*]
  rule match: ([<~='$s;'>? ...] -> @: $@ + 1 + $::length;) <'.'>? -> $@
end index

'ba is found in positions $:'banana' -> index&{s:'ba'}; in banana' -> !OUT::write
'
' -> !OUT::write
'ana is found in positions $:'banana' -> index&{s:'ana'}; in banana' -> !OUT::write
'
' -> !OUT::write
'c is found in positions $:'banana' -> index&{s:'c'}; in banana' -> !OUT::write