def set_at positions, val
      validate_positions(*positions)
      positions.map { |pos| @data[pos] = val }
      update_position_cache
    end
def set_at positions, val
      validate_positions(*positions)
      positions.map { |pos| @data[pos] = val }
      update_position_cache
    end
Change value at given positions
 @param positions [Array<object>] positional values
 @param [object] val value to assign
 @example
   dv = Daru::Vector.new 'a'..'e'
   dv.set_at [0, 1], 'x'
   dv
   # => #<Daru::Vector(5)>
   #   0   x
   #   1   x
   #   2   c
   #   3   d
   #   4   e