  Second Order Operators (SOOs) are database functions which form secondary
queries based on attributes of the objects returned in an initial query; they
can provide powerful methods to investigate complex, multipartite information
graphs. The NASA Astrophysics Data System (ADS) has implemented four SOOs,
reviews, useful, trending, and similar which use the citations, references,
downloads, and abstract text.
  This tutorial describes these operators in detail, both alone and in
conjunction with other functions. It is intended for scientists and others who
wish to make fuller use of the ADS database. Basic knowledge of the ADS is
assumed.
