[Help-glpk] GNU MathProg language reference?

[Balaco Baco]

Hello,

I would like to know of a wikibook, page or any material that explains 
the language GNU MathProg.

I have read in Wikipedia 
(https://en.wikipedia.org/wiki/GNU_Linear_Programming_Kit) that GNU 
MathProg is a subset of AMPL. I could not find any reference there. I 
have use some of the examples that comes with GLPK/glpsol, but it is not 
an easy path to some problems I am trying to solve with it. A reference 
to each construct that the language has would probably suffice what I 
imagine.

Where can I find this reference?