Thanks for that suggestion.  Declare accomplishes the job, although unfortunately algsubs seems very "fragile" and won't work most of the time.  But I can do the substitutions manually.

Actually, the above solution isn't really usable because the number of dependent variables is too large...eg,

t(a,b,c,d,e,f,g,h,i,j,...)

Causing the result to appear highly complicated, instead of just writing " dt / dx".  The reason I'm doing this is because I'm trying to simplify the display of some complex results :/

Nevermind, I figured it out...it's not too hard;  I can just say

diff( 1/t(x) + t(x), x)

Of course, it would be easier if I didn't have to rewrite E with the substitution, but there probably isn't any way to do that

Thanks for explaining that

Thanks!  I just figured it out at the same time!