Having tex(x[1]) --> x_1 and tex(x1) --> x_1 is
chaotic. If you want to typeset x1 as x_1, you
should (have to) use texput
(%i3) load("mactex-utilities.lisp")$
(%i4) texput(x1,"x_1");
(%o4) x_1
Finally, x__1 is bogus TeX. So tex(x_1) --> x__1 is
wrong.
Barton