A simplified version of map.
map [ 1, 2, 3 ]
def x
+ x 3
#
# becomes
#
collect [ 1, 2, 3 ]
+ elt 3
Collect accepts, instead of a function, a block, where elt contains
the current element and idx the current index.
collect [ 'a', 'b' ]
[ idx, elt ]
Iterating blocks are given 3 to 4 local variables.
A block iterating over an array will receive elt (the current element
of the iteration), idx (the zero-based index of the current element),
and len (the length of the array).
A block iterating over an object will receive key (the current string
key), val (the current value), idx (the zero-based index of the
current key/val), and len (the length of the object).