Get unique values in List of Lists in python -

i want create list (or set) of unique values appearing in list of lists in python. have this:

alist=[['a','b'], ['a', 'b','c'], ['a']]

and following:

unique_values=['a','b','c']

i know list of strings can use set(alist), can't figure how solve in list of lists, since set(alist) gets me error message

unhashable type: 'list'

how can solve it?

array = [['a','b'], ['a', 'b','c'], ['a']] result = set(x l in array x in l)

Call