from typing import Iterable


def unique_letters(edges: Iterable) -> str:
	"""Return unique letters in collection of edges

	Args:
		edges (Iterable): Iterable containing edges, i.e. tuples.
		The last element in the tuples is assumed to contain the letter.

	Returns:
		str: String of unique letters
	"""
	return ''.join(set([edge[-1] for edge in edges]))