Concatenation of a sequence of words: cumSum

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

The cumSum command works on strings like it does on expressions by doing partial concatenation (see Section 6.40.26).

- cumSum takes one argument:
  L, a list of strings.
- cumSum(L) returns a list of strings where the element of index k is the concatenation of the strings in L with indices 0 to k.

Example.
Input:

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

Output:

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