NameError

[nafshar]

I am getting a NameError that I do not understand.
It appears the function "combo" is defined in the proper place, so "combo not found" is puzzling to me.

class Solution(object):
    
    def combo(self, l1, l2):
        cs = []
        for i in range(len(l1)):
            for j in range(len(l2)):
                cs.append(l1[i] + l2[j])
        return cs
    
    
    def letterCombinations(self, digits):
        s = combo(["a", "b", "c"], ["d", "e", "f"])
        return s

Error:
NameError: global name 'combo' is not defined
    s = combo(["a", "b", "c"], ["d", "e", "f"])
Line 16 in letterCombinations (Solution.py)
    ret = Solution().letterCombinations(param_1)
Line 39 in _driver (Solution.py)
    _driver()
Line 49 in <module> (Solution.py)