Matching the first occurrence of a header field

Hello.

I want to construct a procmail recipe based on the _first_
occurrence of the Received header field of the message.
How can I detect the _first_ occurrence of a field in
the header?

Regards,

Romildo
-- 
Prof. José Romildo Malaquias               Departamento de Computação
http://iceb.ufop.br/~romildo       Universidade Federal de Ouro Preto
romildo(_at_)iceb(_dot_)ufop(_dot_)br                                           
Brasil
romildo(_at_)uber(_dot_)com(_dot_)br
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