Can someone explain why this is not an error?

[Bernard Helyer]

http://www.digitalmars.com/d/2.0/declaration.html

"In a declaration declaring multiple symbols, all the declarations must 
be of the same type:"

Yet this compiles:

---
    void main()
    {
        immutable a = 3, b = 4.2, c = true;
    }
---

a, b, and c all have different types. Unless you consider the type as 
'type to be inferred'. Can anyone explain this behaviour to me?