[Boost-build] Using variable modifiers problem

Subject: [Boost-build] Using variable modifiers problem
From: Edward Diener (eldiener_at_[hidden])
Date: 2017-02-19 18:40:23

I seem to be missing some aspect of variable modifiers. In
http://www.boost.org/build/doc/html/jam/language.html#jam.language.variables
it gives tells me that variable modifiers must be in the form of $(vm), 
where v is the variable name and m is the modifier. Yet in feature.jam I 
see this code:

rule values ( feature )
{
     return $($(:E=:G=$(feature)).values) ;
}

and I do not understand how it can be syntactically correct since the :E 
does not follow a variable name AFAICS. Clearly I am missing how such an 
expression is being parsed.

This is not a vain inquiry. I am trying to determine why some code is 
failing when a call to feature.values with a particular parameter is 
returning an empty result, but I cannot do so unless I understand what 
is happening in the above jam code.