[metapost] On simulating def with vardef

[Nicola]

Hi,
I used to think that a vardef behaves like a primary because its 
replacement text is surrounded by begingroup/endgroup. But, recently, I 
have tried this:

def foo = x enddef;
foo := 1; % Ok, sets x=1

vardef foovar = endgroup gobbled true x gobble begingroup enddef;
foovar := 2; % Not ok: ! Improper `:=' will be changed to `='.

I conclude that the replacement text of a vardef macro is treated as a 
primary in any case. Is it correct? Is there a way to have a vardef in 
the left hand side of an assignment in some circumstances?

For those who are care, what I would like to achieve is something like:

vardef suffixof prefixof @# =
   endgroup gobbled true @# gobble begingroup
enddef;
vardef prefixof x@# =
   endgroup gobbled true suffixof #@.@ gobble begingroup
enddef;

so that

prefixof x.a := 0;

would set x=0 (I know that I can use 'str', but I would like to know if 
this is possible without it).

Thanks,
Nicola