case.rb

case # Empty!
when cond1
  do_something
when cond2
  do_another_thing
end

efor.rb

for i in 1..4 when i % 2 == 0
    j in 5..8 when j % 4 == 0
  puts "#{i}, #{j}"
end

efor_eq.rb

for i in 1..4
  if i % 2 == 0
    for j in 5..8
      if j % 4 == 0
        puts "#{i}, #{j}"
      end
    end
  end
end

for.rb

i = 0

[1,2,3].each do |i|
end
p i   #=> 0

for i in [1,2,3]
end
p i   #=> 3

parse.y

primary : k_for
          loops
          k_end
            {
              $$ = $2;
            }

loops   : for_var keyword_in expr_value
          keyword_when expr_value term
          compstmt
            {
              scope = NEW_NODE(NODE_SCOPE,
                table, NEW_IF(cond($5), $7, 0), args);
              $$ = NEW_FOR(0, $3, scope);
            }
        | for_var keyword_in expr_value
          keyword_when expr_value term
          loops
            {
              // 上と同じ。
            }