Concatenation of a sequence of words : cumSum

cumSum works on strings like 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 string of index k, with the strings of indices 0 to k - 1 .
Input :

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

Output :

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