MathP is not CL Standards Compliant

Bad news! MathP, my very own mathematics notation library for Common Lisp, breaks the ANSI CL standard. The problem is that unread-char cannot be called consecutively without calling something that will read a character (like read-char). From the CL Hyperspec page for unread-char:

It is an error to invoke unread-char twice consecutively on the same stream without an intervening call to read-char (or some other input operation which implicitly reads characters) on that stream. 

I used it to basically peek multiple characters ahead, like peek-char, but for multiple characters. This may be why MathP doesn't work in CLISP. That allows me to have arbitrary operators. For example, I could have the following as distinct operators:

• * for multiplication
• ** (perhaps for exponentiation)
• .* (perhaps for element-wise multiplication, like in MATLAB)
• *. (dot product?)
• *.* (perhaps for element-wise exponentiation)
• etc.

I thought I was being clever, but such a use contravenes the standard pretty clearly. So I'm interested in alternatives. Does anyone know if it's possible to do multiple unread-chars with other stream implementations? Also, can you read lisp from them with standard reading functions?