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{\bf\LARGE
Acknowledgments}
\end{center}

This dissertation would not have been possible without the help and
support of many different people. First and foremost my thesis
advisor, Frank Pfenning, who has guided me through every step of
this research, from the high level design decisions down to the most
technical details. There is probably not a single line that does not
carry his imprint. Of course, any remaining mistake is solely my 
responsibility. In the process, he has taught me a rigorous
discipline and a style in approaching formal problems that has made me a
far better researcher and perhaps a better person, too.

I also would like to thank wholeheartedly the members of the
dissertation committee, Dale Miller and Dana Scott, for their useful
comments and encouragement.

I am also deeply grateful to Carsten Sch{\"u}rmann, who, under the
motto "What do you want to prove today?" has always been ready and
willing to discuss a wide range of issues from from type-theory to the
practicalities of the Twelf system. I'm also indebted to Roberto Virga
for many discussions and his precious help in debugging the strict
calculus rules. Thanks to Iliano Cervesato for his comments to an
early version of the material in Chapter~\ref{ch:strict} and for
making available his manuscript~\cite{Cervesato99pllf}.

I'd also like to thank Mario Ornaghi for his friendship and
hospitality at DSI, Milan and the many discussions on the essence of
logic programming. Thanks also to Ugo Moscato for his support at DSI.

In the health department, I want to thanks from the
deep of my heart Dr. Garfinkel, Dr. Velletri, Dr. Oliva, Dr. Zigler,
Jean Bender and the wonderful people at GAV.

I wouldn't have been able to survive all those years in Pittsburgh
without the company of the local Italian community and associates:
Barbara and Jerry, Cristina and Massimo, Violetta and Ben, Cristina
and Dude, Ulrike and Massimo, Stefano and Kathy, Giovanna and Aldo. A
particular thanks to Lena.

Finally, I want to thank my beloved Claudia, who has been waiting for
me on the other part of the ocean, so very patient and affectionate
during those hard and long years of separation.

It is so sad that my mother is not here anymore to witness the happy
conclusion of such a difficult journey. All I can do is dedicate this
dissertation to her memory.
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