[metapost] New MPlib beta

[Karl Berry]

    Do you happen to know the exact logic web2c uses, by any chance?

You mean so you can reimplement it?  Duplicating all that code sounds
highly problematic to me.

Anyway (off the top of my head), if -mem is specified, that is used.  If
&foo is specified on the cmdline or first line of the input file, that
is used.  Otherwise, $0.mem is used.  See mpost --help :).

Probably you meant something something more complicated  ...