This checkout is s4-fieldalgo, a fork of victorliu/S4 1.1.  The import name is
still S4 and existing scripts still run; see CHANGELOG-fieldalgo.md for what
this line changed, and validation/UPSTREAM.md for the defects it fixes.
Upstream's own README follows.

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S4: Stanford Stratified Structure Solver (http://fan.group.stanford.edu/S4/)

A program for computing electromagnetic fields in periodic, layered
structures, developed by Victor Liu (victorliu@alumni.stanford.edu) of the
Fan group in the Stanford Electrical Engineering Department.

See the S4 manual, in doc/index.html, for a complete
description of the package and its user interface, as well as
installation instructions, the license and copyright, contact
addresses, and other important information.