Making one struct work in place of another for function calls.

[Liam McGillivray]

I have two structs that serve roughly the same purpose, and I 
would like one to be accepted when the other is declared as a 
function parameter.

To better understand what I mean, take the following example, 
where I have a function, and two structs.
```
struct typeA {
     // Some member variables here
}

struct typeB {
     // Some similar member variables here, but in a different 
format
}

float someFunction(typeB input) {
     // Does some stuff
     // Returns result
}
```

If I want to be able to call `someFunction` (or any function with 
`TypeB` as a parameter) using `TypeA` in place of `TypeB`, and 
I'm willing to modify the definition of `TypeA`, I know that I 
can add an `opCast` and `alias this = opCast!TypeB` to `TypeA`.

But what if `typeA` is in an external library? Is there any way I 
can get `someFunction` (and any function with a `typeB` 
parameter) to accept `typeA`, only modifying the definition of 
`TypeB` (and possibly adding a single global line in it's module)?