The program makeindex is a general purpose hierarchical index
generator; it accepts one or more input files (often produced by a
text formatter such as TeX or troff, sorts the entries, and produces
an output file which can be formatted.  The index can have up to three
levels (0, 1, and 2) of subitem nesting.  The way in which words are
flagged for indexing within the main document is specific to the
formatter used; makeindex does not automate the process of selecting
these words.  As the output index is hierarchical, makeindex can be
considered complimentary to the awk-based make.index system of Bentley
and Kernighan, which is specific to troff, generates non-hierarchical
indices, and employs a much simpler syntax for indicating index
entries.