Concatenation of a sequence of words : cumSum

### 2.4.3 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?`