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sum 

        # Array.sum

(from ruby core)
---
    array.sum(init = 0) -> object
    array.sum(init = 0) {|element| ... } -> object

---

When no block is given, returns the object equivalent to:
    sum = init
    array.each {|element| sum += element }
    sum

For example, `[e1, e2, e3].sum` returns `init + e1 + e2 + e3`.

Examples:
    a = [0, 1, 2, 3]
    a.sum # => 6
    a.sum(100) # => 106

The elements need not be numeric, but must be `+`-compatible with each
other and with `init`:
    a = ['abc', 'def', 'ghi']
    a.sum('jkl') # => "jklabcdefghi"

When a block is given, it is called with each element and the block's
return value (instead of the element itself) is used as the addend:
    a = ['zero', 1, :two]
    s = a.sum('Coerced and concatenated: ') {|element| element.to_s }
    s # => "Coerced and concatenated: zero1two"

Notes:
*   Array#join and Array#flatten may be faster than Array#sum for an
    Array of Strings or an Array of Arrays.
*   Array#sum method may not respect method redefinition of "+" methods
    such as Integer#+.

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