Be careful with ruby 'Hash.merge!'

2.2.4 :001 > def doit(opts)
2.2.4 :002?>     opts[:foobar] = 1
2.2.4 :003?>   end
 => :doit 
2.2.4 :004 > 
2.2.4 :005 >   opts = {}
 => {} 
2.2.4 :006 > doit(opts)
 => 1 
2.2.4 :007 > p opts
{:foobar=>1}
 => {:foobar=>1}