[metapost] accessing variables in nested macros

[Piotr Zielinski]

On 10/02/07, Stephan Hennig <mailing_list at arcor.de> wrote:
> One additional observation: At least, arguments seem to be evaluated
> before substitution and operations involving a parameter are magically
> chosen sensibly.  [...] > How is the implicit binary operation
> (multiplication) preserved when "a" isn't a variable with a known type
> (numeric)?

I'm not sure here.  Yes, if you use "expr", then "a" is evaluated
before the function is executed, and the result of this evaluation is
substituted for all occurrences of "a".  Judging from MetaPost error
messages, the evaluated parameters are known as (EXPR0), (EXPR1), etc.
The term "textual substitution" is probably not good here, "token
substitution" is better.  Again, I'm not sure here.

Piotr