I have sagecells on a page that are used to solve some equations. In a further cell, I want to substitute a given value in place of a free variable. Sage's free variables have names such as r1, r2 etc.
Is there a way (some kind of wildcard) to substitute a value for a free variable without knowing its name? I'd like this because depending on the order of execution a user would press the cells, the names would change.
It would look like this
CELL1
sol1=solve([x+y==1],x,y)
CELL2
sol2=solve([x-y==3],x,y)
and then further down I would like a new cell that would substitute the free variable in sol1 to a given value, say 0.
However if I use something along the line of
sol1part=sol1.substitute(r1=0) and the user executed for some reason cell2 before cell1, it will not work.