🚀 go-pugleaf

What are the rules regarding circular imports? For example, the following
code must be invalid because 'a' is never declared.

DEFINITION MODULE file1;
FROM file2 IMPORT a;
END file1.

DEFINITION MODULE file2;
FROM file1 IMPORT a;
END file2.

Yet it seems like the following should be valid because there is no such
conflict.

DEFINITION MODULE file1;
FROM file2 IMPORT b;
VAR a : INTEGER;
END file1.

DEFINITION MODULE file2;
FROM file1 IMPORT a;
VAR b : INTEGER;
END file2.

I haven't been able to find anywhere in Wirth's 4th edition that addresses
this issue. So what are the formal rules in this situation and what techniques
do most Modula-2 compilers use to implement them?

Devon