The following FunctionRange 
```mma
(*a=((1-x) y)/(1-y z)*)
FunctionRange[{{((1 - x) y)/(1 - y z)}, 0 <= x <= 1, y > 1, 
   0 < z < 1}, {x}, a, Reals] // Simplify
```
returns 
```mma
y > 1 && z < 1 && z > 0 && y z != 1 && a y z <= a && a <= y + a y z
```
I'm struggling to see how to illustrate the values of `x, y, z` and the resulting values for `a` that are excluded.

Appreciate any hints or tips.