Source code to determine whether a supernova is quadruply, doubly or singly lensed

The associated file, isit4or2or1.v1.0.zip contains C++ source code for
a program that determines whether an apparent supernova in the
vicinity of a quadruply lensed galaxy is one of 4 images or just 2 or
only 1.  It takes as input a model for the gravitational lens and
predicts the positions, magnifications and differences in arrival
times from those images.  These can be used to coordinate follow-up
exposures and analyze archival exposures to measure the fluxes of all
images at a common phase, ideally peak brightness.

A Makefile is included, as is input data taken from an actual
quadruply lensed supernova.  The program, "isit4or2or1," projects the
observed transient back in time to the "diamond caustic" -- the
boundary between quadruply lensed sources and doubles.  The input and
output imitate Keeton's "gravlens," whose notation we adopt.
"Gravlens" is inclusive by design, but "isit4or2or1" exclusively uses the
Witt-Wynne geometric lens model, which accelerates computation.

The tar file includes a README file with instructions for use
of the program.  This code is submitted in conjunction with a
paper submitted to the AAS journals, "The Quadruply Lensed Supernova
SN 2025wny: Implications for LSST", that proposes a protocol for
ancillary observations and archival investigations to extract maximum
benefit from similar systems.