If your project has multiple
.bigbinput files, you can share Mathematics definitions across all files by adding them to the
ourbigbook.texfile on the toplevel directory.
For example, ifthen from any
.bigbfile we in the project can use:
$$ \foo $$
Note however that this is not portable to OurBigBook Web and will likely never be, as we want Web source to be reusable across authors. So the ony way to avoid macro definition conflicts would be to have a namespace system in place, which sounds hard/impossible.
Ideally, you should only use this as a temporary mechanism while you make a pull request to modify the built-in math macros :-)