In particular, I want to write an extension method for this type:

```
type Frame<'TRowKey, string when 'TRowKey : equality> with
  member frame.someMethod =
    // code

```

With that code, I'm getting this error:

> 
> Unexpected identifier in type name. Expected infix operator, quote symbol or other token.
> 
> 
> 

replacing `string` with `String` gives the same result.

The original type is `Frame<'TRowKey, 'TColumnKey (requires equality and equality)>` from the Deedle library.