Deciding one member of iteration chain at runtime

[Chris Piker]

Hi D

I have a main "loop" for a data processing program that looks 
much as follows:
```d
sourceRange
   .operatorA
   .operatorB
   .operatorC
   .operatorD
   .operatorE
   .operatorF
   .operatorG
   .operatorH
   .copy(destination);
```
Where all `operator` items above are InputRange structs that take 
an upstream range, and the pipeline really is 9 operations deep.

In order to handle new functionality it turns out that operatorG 
needs to be of one of two different types at runtime.  How would 
I do something like the following:

```d
auto virtualG;  // <-- probably invalid code, illustrating the 
idea
if(runtime_condition)
    virtualG = operatorG1;
else
    virtualG = operatorG2;

sourceRange
   .operatorA
   .operatorB
   .operatorC
   .operatorD
   .operatorE
   .operatorF
   .virtualG
   .operatorH
   .copy(destination);
```
?

I've tried various usages of `range.InputRangeObject` but haven't 
been able to get the syntax right.  Any suggestions on the best 
way to proceed?  Maybe the whole chain should be wrapped in 
InputRangeObject classes, I don't know.

Thanks,