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so easy it seemed Once found, which yet unfound most would have thought Impossible. (John Milton, 1608 -1674) There are essentially two types of books on a scientific subject: in the first one several authors contribute their specialized approaches to parts of the field in question, which then are edited and compiled to yield a comprehensive and authoritative account. In the second type of book a single author tries to pre sent a view from an individual standpoint which might lead to a more balanced and homogeneous source of information. Both kinds have their merits and de ficiencies. I decided to write this book as a monolithic piece of work for several rea sons. Of course, there was the challenge of coping with the many problems of such an undertaking due to the fact that this field has grown tremendously during the last decades. In addition, being heavily involved in linear oligopyr role chemistry for nearly two decades, it seemed worthwile to prepare a more unifying approach. The request of several colleagues from abroad to give an account in English also triggered this endeavor since most of the work of my group has been published in German.

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