Somewhere between a bug or feature:

[Han]

(11-13-2014 11:31 PM)Mark Hardman Wrote:  

(11-13-2014 10:43 PM)Han Wrote:  A few more trials leads me to think that perhaps you're onto something.

b:=x^2;
b+2*x|x=3; --> x^2+6
b|x=3; ---> 9

I can't explain this.

How is this?

b(x):=x^2
b(x)+2*x|x=3 --> 15

That gets the job done if one wants to get the entire substitution. However, it does not explain how the | function has been implemented. In the case of b|x=3, the substitution seems to uncover one level of abstraction. That is, it substitutes x=2 into the value of b. On the other hand, b+2*x|x=3 only substitutes at face value to b+6, and then evaluates b+6 as x^2+6.