Access documents by research topic. Word searches are available through 11 categories of subjects that recur systematically in the Opera sources, presented in the menu of the left navigation bar.

This second search mode by subjects, which supplements the guided research structure, enables the user to find all the words present in the specification text strings compiled for each category.

It should be noted that specification texts are available only for those cases in which the sources themselves furnish information beyond that already implicit in the basic hierarchical structures of guided research.