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local dictionary = {}
 
local function insertWord(dict, word)
    local head, tail = word:upper():match('^(%a)(%a*)$')
    if not head then return false; end
    if not dict[head] then dict[head] = {}; end
    if tail == '' then
        dict[head][''] = true
        return true
    else
        return insertWord(dict[head], tail)
    end
end
 
local function confirmWord(dict, word)
    local head, tail = word:upper():match('^(%a)(%a*)$')
    if not head then return false; end
    if dict[head] then
        if tail == '' then
            return dict[head][''] or false
        else
            return confirmWord(dict[head], tail)
        end
    else
        return false
    end
end
 
insertWord(dictionary, 'BAG')
 
print(confirmWord(dictionary, 'BAG'))