\midheading{$HH$ production, processes 601--602}
These processes represent the production of a pair of Higgs bosons.
The production proceeds through gluon-fusion one-loop diagrams involving loops
of top quarks. The formulae implemented in the code are taken from ref.~\cite{Glover:1987nx},
where the two Higgs bosons are treated as being on-shell. To enforce this
condition, the code sets zerowidth to true, overriding the value set in the input file.
The calculation can be performed at LO only, (i.e.\ one-loop order only).
Two decays of the Higgs bosons are currently foreseen, although other decays can easily be implemented.
In process {\tt 601}, one Higgs boson decays to
a pair of $b$-quarks, and the other decays to a pair of $\tau$'s.
In process {\tt 602}, one Higgs boson decays to
a pair of $b$-quarks, and the other decays to a pair of photons.