[metapost] multiple and nested input

[Josef Eschgfaeller]

I have some difficulties with the input command
in Metapost:

(1) Can one use only one of them in each file?
      It seems that a second input has no effect.

(2) Can inputs not be nested (in several files)?

Example for (1): If I want a definition in file B,
after "input A; input B;" it is not available, but
it becomes so if I write "input B; input A;".

In general: Where can one read such syntax
or grammar details? They are not contained
in the books or tutorials I know (Hobby,
Grogono, Entenmann, Latex companion,
Hoenig, ...).

Thanks
Josef Eschgfaeller