Alpha blending Shader - Use property in ConstantColor (?)

Hi,

I am a total shader noob and started looking at some shaderlab reference pages and tutorials a view minutes ago.
I wanted a simple (?) alpha blending shader for my 2D game (it's going to be applied to planes).

That's what I got:

Code (csharp):

1. Shader "Custom/Diffuse Alpha" {
2.     Properties {
3.         _MainTex ("Base (RGB)", 2D) = "white" {}
4.         _Alpha ("Alpha Value", Range (0.00, 1.00)) = 1.00
5.     }
6.    
7.     SubShader
8.     {
9.         Tags { Queue = Transparent }
10.        
11.         Blend SrcAlpha OneMinusSrcAlpha
12.        
13.         Pass
14.         {
15.             SetTexture[_MainTex]
16.             {
17.                 constantColor(1.0,1.0,1.0,_Alpha)
18.                 combine texture, texture * constant
19.             }
20.         }
21.     }
22.    
23.     FallBack "Diffuse"
24. }
25.  

This works great, BUT it won't let me use the _Alpha property to specify the constant color :-(
Why? :-(

It feels like I'm THIS close to the solution. Is it just a little syntax thing?

Edit: OK I got it.

It's just

Code (csharp):

1. constantColor(1.0,1.0,1.0, [_Alpha])