global

elements specified in the range of indexes specified by +interval+.
positions it occupies in the Enumerable, optionally restricted to the
Returns a hash mapping each element of an Enumerable to the set of

def position_hash(enum, interval)
  string = enum.is_a?(String)
  hash = Hash.new { |h, k| h[k] = [] }
  interval.each do |i|
    k = string ? enum[i, 1] : enum[i]
    hash[k] << i
  end
  hash
end