class Hash

def reverse_merge(other_hash)

with default values.
This is particularly useful for initializing an options hash

options = { size: 25, velocity: 10 }.merge(options)

is equivalent to

options = options.reverse_merge(size: 25, velocity: 10)

Merges the caller into +other_hash+. For example, 
def reverse_merge(other_hash)
  other_hash.merge(self)
end