  Aspects ([asp{\epsilon}], "ASsociation PositionnellE/ProbabilistE de
CaTalogues de Sources" in French) is a Fortran 95 code for the
cross-identification of astrophysical sources. Its source files are freely
available.
  Given the coordinates and positional uncertainties of all the sources in two
catalogs K and K', Aspects computes the probability that an object in K and one
in K' are the same or that they have no counterpart. Three exclusive
assumptions are considered: (1) Several-to-one associations: a K-source has at
most one counterpart in K', but a K'-source may have several counterparts in K;
(2) One-to-several associations: the same with K and K' swapped; (3) One-to-one
associations: a K-source has at most one counterpart in K' and vice versa.
  To compute the probabilities of association, Aspects needs the a priori (i.e.
ignoring positions) probability that an object has a counterpart. The code
obtains estimates of this quantity by maximizing the likelihood to observe all
the sources at their effective positions under each assumption. The likelihood
may also be used to determine the most appropriate model, given the data, or to
estimate the typical positional uncertainty if unknown.
