[exim] How to use $?

    I must be dumb or am blindly missing it in the manual, but how do I
escape the $ character to use it in say, checking for files that end with
the .$$$ extension the demime verb?  This, for example, is wrong:

demime = $$$:bat:chm:cmd:com:exe:pif:scr:shs:vbe:vbs

Thanks,
Chris