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The callback interface seems to have been  added in readline 2.1, 
from 1997. There seem to be configure tests in the current 
Modules/readline.c code to search for features new to readline 2.1  
so my current approach would be upping the minimum readline 
version from 2.0 to 2.1. If needed I could test for the callback 
interface and use it if available, but fall back to the readline() 
interface otherwise (and leave the thread and signal handling 
issues in place when used with older readline.)