Evaluate a primitive at boundaries: preval

### 2.12.18 Evaluate a primitive at boundaries: `preval`

`preval` takes three arguments : an expression `F` depending on the variable `x`, and two expressions `a` and `b`.
`preval` computes F|x=b−F|x=a.
`preval` is used to compute a definite integral when the primitive F of the integrand f is known. Assume for example that `F:=int(f,x)`, then `preval(F,a,b)` is equivalent to `int(f,x,a,b)` but does not require to compute again `F` from `f` if you change the values of a or b.
Input :

`preval(x```^```2+x,2,3)`

Output :

`6`