I have used the open source maxima quite a bit, and variable replacement was easy. For example:
I could combine very easily, like:
and so on...I use variable replacement in equations VERY regularly, and so far, all I have found is having to use a mouse and select "Evaluate at a point". I'm not even sure I could put other equations in there if I wanted to. Even if it does work, the more complicated scenarios that I did routinely in maxima would still be virtually impossible this way.
Thus, the question is, is there doc somewhere on how to make various variable replacements, and equation combining like this easy?