[metapost] Bug? Parsing def := := enddef;

[Nicola]

Hi,
according to The METAFONTbook, a macro definition can be 'def' followed 
by a symbolic token followed by either = or :=. The following is the 
output of a couple of mpost sessions:

This is MetaPost, version 1.208 (kpathsea version 3.5.7dev)
**\relax

*def := = enddef;

*showtoken :=;
> :==macro:

But the following gives an error:

This is MetaPost, version 1.208 (kpathsea version 3.5.7dev)
**\relax

*def := := enddef;
! Missing `=' has been inserted.
<to be read again> 
                   :=
<*> def := :=
              enddef;
? 

That looks like a small bug. Taco, can you confirm?

Nicola