output

[arup@Ruby]$ ruby a.rb
a.rb:7: syntax error, unexpected keyword_do_block
  do |y|
    ^
a.rb:10: syntax error, unexpected keyword_end, expecting end-of-input
[arup@Ruby]$

seq.rb

def seq(first, size)
  e = Enumerator.new(yield(first))
  e.take(size)
end

res = seq(5, 4) do |n| 
  do |y|
    loop { y << n; n = 2*n - 3 }
  end
end

p res