How to evaluate A Taylor series at a specific value

[parisse]

| is evaluated more like a program, where x would be like an argument. taylor(sin(x))|x=0.5 will evaluate sin(x) to sin(0.5) before taylor is called.
It's hard to evaluate |. You don't want to get sin(1) for sin(x)|x=2 if x:=1; has been run, yet you want to get sin(6) for sin(a*x)|x=2 if a:=3.
I don't like this notation, I prefer subst.