[PROJ] "eck_greif" projection, "libproject"?

[Nyall Dawson]

Hey list,

Does anyone know what the "+proj=eck_greif" mentioned in section 7.2.1
of http://priede.bf.lu.lv/ftp/pub/TIS/bibliotekas/PROJ.4/libproj4_manual.pdf
refers to? I can't find any reference to this in master proj code base
at all. (It's also referenced here:
https://github.com/Esri/projection-engine-db-doc/blob/master/other/proj4-esri.txt#L154,
which is why I'm asking the question)

The proj4-esri.txt file also states:

"Names marked with a * are only in the libproject distribution
Names marked with a @ are only in the PROJ4      distribution"
(https://github.com/Esri/projection-engine-db-doc/blob/master/other/proj4-esri.txt#L21)

Does anyone have any clues as to what "libproject distribution" is? I
can't find anything relevant from my searching, but given the context
it sounds like some historic proj fork?

Nyall