tt(IFSTRSUB) tests whether a string is a sub-string of another 
string. It acts similar to link(IFSTREQUAL)(IFSTREQUAL), but
it tests whether the second string is part of the first one. 

    The first two arguments of tt(IFSTREQULA) are partially evaluated:
    itemization(
    it() If the argument is the name of a symbol, the symbol's value is the
value of the argument
    it() Otherwise, the argument itself is used.
    )

In the degenerate case where the string to be compared is actually the name of
a tt(SYMBOL), use a temporary tt(SYMBOL) variable containing the name of that
symbol, and compare it to whatever you want to compare it with. Alternatively,
write a blank space behind the arguments, since the arguments are then
interpreted `as is'. In practice, the need for these constructions seem to
arise seldom, however.

    Example:
    verb(
        IFSTRSUB(haystack)(needle)(
            `needle' was found in `haystack'
        )(
            `needle' was not found in `haystack'
        )
    )
    Note that both `haystack' and `needle' may be the names of symbols. If
they are, their contents are is compared, rather than the literal names
`haystack' and `needle'
    
