The survey package has benefited from code, bug reports, and feature 
requests from users:
  Daniel Almirall
  Stefano Calza
  Daniella Gollinelli
  Gillian Raab
  Fred Rohde
  Takahiro Tsuchiya  
  Ana Quiterio
  Renzo Vettori
  Steve Roberts
  Andrew Robinson
  Lee Sieswerda
  Tobias Verbeke
and from initial dicussions with Chris Wild and Alastair Scott.
