### 5.4.3 Concatenation of a sequence of words : cumSum

cumSum works on strings like it does on expressions by doing partial concatenation.
cumSum takes as argument a list of strings.
cumSum returns a list of strings where the element of index k is the concatenation of the strings with indices 0 to k .
Input :

cumSum("Hello, ","is ","that ","you?")

Output :

"Hello, ","Hello, is ","Hello, is that ","Hello, is that you?