Function freshVarGenerator

freshVarGenerator(existing: Iterable<string>, prefix?: string): () => Variable
Creates a generator of fresh (non-colliding) RDF variables, coining names from a monotonically increasing index and remembering every name it hands out.
Parameters
- existing: Iterable<string>
  Variable names that already exist within the operation tree
- prefix: string = 'v_'
  Prefix used for the coined variable names
Returns () => Variable
a function that returns a new, unused variable on each call
Example
```
const fresh = freshVarGenerator([ 'x', 'v_0' ]);
fresh(); // ?v_1  (v_0 was taken)
fresh(); // ?v_2
```
- Defined in lib/utils.ts:47