How to use "char" for combinated keys

[lyl]

A key value(integer) can be obtained by the built-in function "char".

For example,

Code Select Expand

(char "a")          → 97
My question is: How to  obtain the value of combinated keys like this:

(myfunc "ctrl a") which means "CTRL + a" . And also  "CTRL+Alt+a" , "CTRL+SHIFT+a" are wanted.

Thank you.