```what about having map in standard (and map_index too)?

sorry to bother you... but bash me for any bad style...

--------------------------------------------------------------------------------
class Array
def map
dup.filter{|i| yield(i) }
end
def map_index
l = []
each_index{|i| l += yield(i, self[i]) }
l
end
def fold_left
a = self[0]
self[1..-1].each{|i| a = yield(a, i) }
a
end
def sum; fold_left{|i,j| i+j}; end
end

def fold_left(f, *l)
a = l.shift
l.each{|i| a = f.call(a, i) }
a
end
def sum1(*l); fold_left(proc{|i,j| i+j }, *l); end

def fold_left2(*l)
a = l.shift
l.each{|i| a = yield(a, i) }
a
end
# is same as
def fold_left2(*l, &f)
a = l.shift
l.each{|i| a = f.call(a, i) }
a
end
def sum2(*l); fold_left2(*l){|i,j| i+j }; end

def sum3(*l); l.fold_left{|i,j| i+j}; end

p sum1(1, 2, 3)
p sum2(1, 2, 3)
p sum3(1, 2, 3)
p [1, 2, 3].sum

def mapn(*l)
l[0].map_index{|i,_e| yield(l.map{|e| e[i]}) }
end

def zip(*l); mapn(*l){|l| [l]}; end
def unzip(l); mapn(*l){|l| [l]}; end

p l = zip(a = [1,2,10], b = [1,2,222])
(a2,b2) = unzip(l)
p a==a2, b==b2
--------------------------------------------------------------------------------
```