Write a comparator for a list of phonetic words for the letters of the [greek alphabet](https://en.wikipedia.org/wiki/Greek_alphabet).
A comparator is:
> *a custom comparison function of two arguments (iterable elements) which should return a negative, zero or positive number depending on whether the first argument is considered smaller than, equal to, or larger than the second argument*
*(source: https://docs.python.org/2/library/functions.html#sorted)*
The greek alphabet is preloded for you as `greek_alphabet`:
```python
greek_alphabet = (
    'alpha', 'beta', 'gamma', 'delta', 'epsilon', 'zeta', 
    'eta', 'theta', 'iota', 'kappa', 'lambda', 'mu', 
    'nu', 'xi', 'omicron', 'pi', 'rho', 'sigma',
    'tau', 'upsilon', 'phi', 'chi', 'psi', 'omega')
```
## Examples
```python
greek_comparator('alpha', 'beta')   <  0
greek_comparator('psi', 'psi')      == 0
greek_comparator('upsilon', 'rho')  >  0
```
def greek_comparator(l, r):
    gt = (
        'alpha', 'beta', 'gamma', 'delta', 'epsilon', 'zeta', 
        'eta', 'theta', 'iota', 'kappa', 'lambda', 'mu', 
        'nu', 'xi', 'omicron', 'pi', 'rho', 'sigma',
        'tau', 'upsilon', 'phi', 'chi', 'psi', 'omega')
    return gt.index(l)- gt.index(r)
def greek_comparator(l, r):
    gt = (
        'alpha', 'beta', 'gamma', 'delta', 'epsilon', 'zeta', 
        'eta', 'theta', 'iota', 'kappa', 'lambda', 'mu', 
        'nu', 'xi', 'omicron', 'pi', 'rho', 'sigma',
        'tau', 'upsilon', 'phi', 'chi', 'psi', 'omega')
    return gt.index(l)- gt.index(r)