Patrick Blackburn and 
   Maarten de Rijke. Why Combine Logics? In  Studia Logica. An International Journal for Symbolic Logic, Vol. 59(1):5-27,   1997. Note: but published later.
 @Article{Blackburn_de Rijke:1997,
        AUTHOR = {Blackburn, Patrick and de Rijke, Maarten},
        TITLE = {Why Combine Logics?},
        YEAR = {1997},
        JOURNAL = {Studia Logica. An International Journal for Symbolic Logic},
        VOLUME = {59},
        NUMBER = {1},
        PAGES = {5-27},
        NOTE = {but published later} } 
     |