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[Arno K]

you perhaps should use: y:=unapply(lagrange(lstA,lstB),x), and then y(xVal),
this at least works for the command line from cas, from home that is another question, the output then given by y:=unapply(lagrange(listA,listB),X) is some kind of weird: program(X,[0],X+1) and y(6) delivers Error: Invalid input, but in a cas program the solution above should work.
I know, this is not what you want to do, but with the prime one often has to use, well, let me call it, workarounds to get something done. I would say ist a bug in the where function, as in simple instructions like x+1|x=6 it works correctly in cas (lower case x) and home (upper case x).
Arno