sum

        # Range.sum

(from ruby core)
### Implementation from Enumerable
---
    sum(initial_value = 0)                  -> number
    sum(initial_value = 0) {|element| ... } -> object

---

With no block given, returns the sum of `initial_value` and the
elements:

    (1..100).sum          # => 5050
    (1..100).sum(1)       # => 5051
    ('a'..'d').sum('foo') # => "fooabcd"

Generally, the sum is computed using methods `+` and `each`; for
performance optimizations, those methods may not be used, and so any
redefinition of those methods may not have effect here.

One such optimization: When possible, computes using Gauss's summation
formula *n(n+1)/2*:

    100 * (100 + 1) / 2 # => 5050

With a block given, calls the block with each element; returns the sum
of `initial_value` and the block return values:

    (1..4).sum {|i| i*i }                        # => 30
    (1..4).sum(100) {|i| i*i }                   # => 130
    h = {a: 0, b: 1, c: 2, d: 3, e: 4, f: 5}
    h.sum {|key, value| value.odd? ? value : 0 } # => 9
    ('a'..'f').sum('x') {|c| c < 'd' ? c : '' }  # => "xabc"