The tt(IFSTRSUB) macro tests whether a string is a sub-string of another 
string. It is similar to tt(IFSTREQUAL) (see section ref(ifstrequal)), except 
that the test is whether the second string is part of the first one. Hence:

verb(\
IFSTRSUB(some piece of text)(ce o)
    (truelist)
    (falselist))
    
evaluates to tt(truelist).
