convert _ _ to ital etc.

[Bill Troop bill@xxxxxxxx]

My xpl skills are now so rusty that I have no idea
how to write a simple program that would go
through a document and convert the first _ to a
mdit (for example, or a mdul), the next to a mdnm,
the next to a mdit, the next to a mdnm, and thus cycle
through the document until the end. The idea being
to convert a simple email-language document,
where emphasis is specified by pairs of _ or *,
into ordinary XyW formatting. Could someone
indulge me, please?