9.1.11  Accumulating terms: accumulate_head_tail

The accumulate_head_tail command replaces the first terms of a list by their sum and the last terms of a list by their sum.

• accumulate_head_tail takes three arguments:
  • L, a list of numbers.
  • n, the number of initial terms to add.
  • m, the number of end terms to add.
• accumulate_head_tail(L,n,m) returns the list with the n initial terms and m end terms replaced by their sums.

Example.
Input:

Output: