Searches a set <metadata> to find all relations <rel>
    Returns a list of the values of those relations
    (A list, because a rel can occur more than once)

def _values(metadata, rel):
    """Searches a set <metadata> to find all relations <rel>
    Returns a list of the values of those relations
    (A list, because a rel can occur more than once)"""
    result = []
    for r in metadata:
        if(r[REL] == rel):
            result.append(r[VAL])

    return result