[texhax] Hyphens in math mode

[Karl Berry]

    I need to create a hyphen in an equation without using \hbox. It has 
    to be as simple as possible, Plain.TeX. Is this possible?

Do you mean, just print a hyphen character from the current roman font?
How about this:  

$ ... \the\textfont0 \char`\- ... $

best,
karl