STIX Two Text (Regular, Italic) and STIX Two Math, unmodified, from the STIX Fonts
Project (https://github.com/stipub/stixfonts), version 2.13 (Text) and 2.12 (Math).
Copyright 2001-2021 The STIX Fonts Project Authors. Licensed under the SIL Open
Font License, Version 1.1: the full text is in OFL.txt, and the fonts' own license
fields say the same.

irspec uses them only when STIX Two is not installed on the machine: matplotlib and
R draw with them, and the plain-lualatex fallback loads them from irspec-fonts/.
They are not sold or distributed on their own; they travel with irspec under the
same license terms.
