Wildcards differ in certain details from the constructs described in the aforementioned
paper, in particular in the use of capture conversion (§5.1.10) rather than the close
operation described by Igarashi and Viroli. For a formal account of wildcards, see Wild
FJ by Mads Torgersen, Erik Ernst and Christian Plesner Hansen, in the 12th workshop on
Foundations of Object Oriented Programming (FOOL 2005).