We describe an extensible constructor tool that helps 
information experts (e.g., librarians) create specialized query 
synthesizers for heterogeneous digital-library environments. 
A query synthesizer provides a graphical user interface in 
which a digital-library patron can specify a high-level, 
fielded, multi-source query. Furthermore, a query synthesizer 
interacts with a query translator and an attribute translator to 
transform high-level queries into sets of source-specific 
queries. We discuss how the constructor can facilitate 
discovery of available attributes (e.g., title), collation of 
schemas from different sources, selection of input widgets 
for a synthesizer (e.g., a text box or a drop-down list widget 
to support input of controlled vocabulary),, and other design 
aspects. We also describe a prototype constructor we 
implemented, based on the Stanford InfoBus and metadata 
architecture.